a method to predict stack performance

MICHAEL P. FITZGERALD

A METHOD TO PREDICT STACK PERFORMANCE

h e s e n t e d at the 22nd Annual Technical Symposium of the Association of Scientists and Engineers of the Naval Sea Systems 1 J LCommand. 29 March 1985.

THE AUTHOR

joined Naval Sea Systems Command in 1984 after receiving a B.S. degree in ocean engineering from the Florida Institute of Technology. He recently joined the surface ship hydrodynamics branch of the hull form and hydrodynamics performance division after working in the stability division, the preliminary design division, the ship arrangements design division, the submarine structural integrity division, the deck and replenishment systems division, the air conditioning and life support division, the collective protection systems project of- fice, and the ocean engineering ofJce. At present he is tem- porarily assigned to the U.S. Naval Academy Hydromechanics Laboratory. He is a member of ASNE. ASE, and SNAME.

ABSTRACT

This paper describes a method to predict stack gas dispersion Performance that was developed during the design of a recent surface combatant. This method is presented in the form of a step-by-step procedure. This method includes a two-sided procedure for the analysis of model test results. This two-sided approach combines a quantitative approach and a qualitative approach to evaluate model test results. An introduction to stack design considerations and the problems associated with low-profile stack design are presented. The major points of stack design technology are highlighted. Future directions in stack design technology are discussed.

INTRODUCTION

Historically, stack design has not been a major con- cern of naval architects. Since the days of the first steam ships, stacks have been erected to eject engine exhaust high enough to keep the effluent from falling back onto personnel, into engine air intakes, and into ventilation intakes. When in doubt, the designer simply raised the stacks a little higher. In today’s world of advanced radar systems and heat-seeking missiles, the stack plays a larger role in naval ship design.

Figure 1 shows a general overview of the evolution of the factors affecting stack design. The first stacks were pipes designed to keep the smoke and ash exhausted from the coal-fired boilers from falling back onto the early steam-powered ships. The content of the exhausted stack gasses changed during the Navy’s coal-to- oil conversion in the early part of this century. Stack designs began to feature the pipes enclosed in casings to improve structural integrity of the stacks and aesthetic appearance. By the 195Os, a strong push for an aerodynamic solution to stack design was underway. Stack casings and superstructures became more streamlined in order to improve the flow characteristics over the ships. By the mid-l%Os, the gas turbine, with its significantly

increased intake and uptake requirements, emerged as a prime mover for warships. The higher temperatures resulting from gas turbine exhaust, compared with the exhaust from steam or diesel powered ships, pose a threat to temperature-sensitive topside electronic equipment. Model testing methods were developed to determine the boundary of the turbulent zone and to determine the stack performance at various velocity ratios and relative wind headings. Analysis techniques based on aerodynamic theory, model testing, and full-scale trials were developed as design tools. In 1977, these analytical tools were incorporated into a firmly established stack design practice [l] and are reflected in a NAVSEA ship design standard currently undergoing review.

While the tools to design stacks for good stack gas dispersion performance were advancing rapidly, other factors became a driving force in naval ship stack design. Combat considerations such as radar cross-section, infrared signature, and radar blockage limit stack height and shape and therefore compromise stack performance. As a result, today’s stack designer is faced with a dilemma. On one hand, we have an established design technology and highly advanced computer programs to design stacks for good performance. On the other hand, we have stringent combat constraints that compromise the goal of designing stacks for good performance. This conflict was quite evident in the design of a recent surface combatant.

By 1984, the need had arisen to establish a more rig- orous methodology to predict the performance of a “compromised” low-profile stack design. By establishing this methodology, the Navy will be better able to deal with the problems associated with low-profile stacks as they occur in future ship designs. This paper summarizes the results of an initial effort to achieve this goal.

BACKGROUND

The hull form and hydrodynamics performance division designs stack configurations for acceptable gas dispersion performance by using the principles and pro- cedures of “Stack Design Technology for Naval and Merchant Ships.” A quick look at the key points of stack design technology is now in order.

The goal of stack design is to keep the stack gasses from downwashing onto the ship’s decks and to prevent the exposure of temperature-sensitive electronic equipment to high temperatures. The basic cause of stack gas

35 Naval Engineers Journal, September 1986

PREDICTING STACK PERFORMANCE FITZGERALD

1940’s and 1950’s

ENGINE DECK AIR VENTILATION

HAZARD INTAKES INTAKES COST

DESIGN STACKS FOR GOOD DISPERSION PERFORMANCE

EARLY STEAM SHIPS

- DESIGN STACKS FOR GOOD 0 DISPERSION PERFORMANCE

VENTILATION ENGINE COST

1977

STRONG PUSH FOR

AERODYNAMIC SOLUTION

DESIGN STACKS FOR GOOD EXHAUST DISPERSION PERFORMANCE 0 STREAMLINED

STACKS

STRUCTURE AESTHETIC

APPEARANCE ELECTRONIC EQUIPMENT

HAZARDS WEIGHT

m

DECK TOPSIDE INFRARED HAZAROS WEIGHT SIGNATURE ADVANCED

ANALYSIS HELO AND SPACE TECHNIQUES OPS STRUCTURE

COST

1984

0 SMOKE AND ASH 0 CIRCULAR PIPES

TRY TO DESIGN STACKS WITHIN CONSTRAINTS FOR GOOD

DISPERSION PERFORMANCE

EXHAUST 0 TRAPEZOIDAL

STACKS

ELECTRONIC ENGINE STRUCTURE EQUIPMENT INTAKES I -

TOPSIDE HAZARDS WEIGHT STACK

DESIGN TECHNOLOGY

DIESEL EXHAUST CIRCULAR STACKS

1 IR I RCS

PREDICT THE DISPERSION

“IF IN DOUBT, RAISE STACKS A L l l l L E HIGHER!”

“STACK MUST EXTEND ABOVEBOUNDARY OF TURBULENT ZONE.”

“STACK HEIGHT MAY BE ONE EQUIVALENT PIPE DIAMETER BELOW BOUNDARY OF TURBULENT ZONE.”

“CAN WE PUT THE STACKS EVEN LOWER?”

Figure 1. Evolution of the Factors Affecting Stack Design

downwash is boundary layer flow separation behind turbulent region, it is attracted into the turbulent zone bluff obstacles such as deckhouses, bulwarks, masts, by suction due to lower pressure. A severe case of this and similar topside structures exposed to the wind backflow into the turbulent zone can result in com- (Figure 2). When exhaust gas approaches a separated pletely filling this low pressure region with gasses down

36 Naval Engineers Journal, Septembery 1986

FITZGERALD PREDICTING STACK PERFORMANCE

EXHAUST PLUME

BOUNDARY OF TURBULENT ZONE

TURBULENT ZONE

Figure 2. Principle of Acceptable Stack Design

to the deck level [2]. Gas sweepdown can be prevented by ejecting the exhausted gasses into the region above the boundary of the turbulent zone (Figure 2). Gas sweepdown may also be avoided for stack heights below the boundary of the turbulent zone if the stack gas exit velocity is high enough to penetrate the boundary. High temperatures on electronic equipment are prevented by cooling the exhausted gas by use of eductors [l] or by providing a sufficient distance between the stacks and the electronic equipment. Both momentum and buoy- ancy affect the trajectory of the exhausted stack gas plume.

Stack performance is determined separately for each stack performance element (SPE). Typical stack performance elements are temperature-sensitive electronic equipment, gas sweepdown on decks, gas ingestion into turbine air intakes, and gas presence on helicopter operations. Stack performance varies significantly with velocity ratio (Figure 3) and relative wind-over-deck heading (Figure 4). Higher velocity ratios provide better performance than lower velocity ratios. Best overall performance usually occurs at 0 degrees (headwinds) relative wind heading. Performance can become significantly degraded at about 15 degrees port or starboard as the

MODEL TESTS ARE BASED UPON SCALING OF VELOCITY RATIO

V, STACK GAS EXIT VELOCITY

V, RELATIVE WIND VELOCITY VELOCITY RATIO = ~ =

- Figure 3. Velocity Ratio Diagram

Figure 4. Vector Diagram for Relative Wind Velocity

stack casings streamlined for headwinds become, in effect, no longer streamlined [3]. Model testing and math- ematical modeling are used to determine the performance at various conditions. A computer program is used to predict the probabilities of the conditions at which unsatisfactory performance will occur. A more in-depth look at stack design technology is provided in References [l], [2], [31, and [41.

METHODOLOGY TO PREDICT STACK PERFORMANCE

The stack gas dispersion performance of a stack configuration design is predicted by a method consisting of eleven major steps. This method is illustrated in Figure 5 and described as follows. An example application of this methodology is presented in Appendix A.

DETERMINATION OF THE “AVERAGE OPERATING ENVIRONMENT”

The computer program PROBVR is used by the hull form and hydrodynamics performance division to generate the probabilities of encountering specific velocity ratios in specific sectors for a ship operating in any of the following nine geographic environments:

1) Northern NE Atlantic 2) Northern North Atlantic 3) Eastern Mediterranean 4) Sea of Japan 5 ) Gulf of Aden 6) SE North Atlantic 7) North Pacific 8) The Caribbean 9) Strait of Malacca

But which environment is the most representative of what the ship will actually encounter? Past designs used the Caribbean, which was considered an “average” environment for stack performance. In a recent design, results from both the Caribbean and the Northern



\L ESTABLISH CRITERIA FOR EACH STACK 111

PERFORMANCE ELEMENT 1 STACK PERFORMANCE ELEMENT

I 1 DETERMINE ALTERNATE STACK CONFIGURATIONS TO BE EXAMINED

\L PERFORM PLUME TRAJECTORY AN0 VI

TEMPERATURE CALCULATIONS 1 MOOEL

VII <>-- NEEDED? TESTING

YES

PLAN MODEL TESTING Vlll I 1

\L

\L

\L

PERFORM MODEL TESTING IX

I PERFORM ANALYSIS OF MODEL TEST RESULTS I [ PREPARE CONCLUSIONS A-k

Figure 5. Flowchart of Method to Predict Stack Performance

North Atlantic were examined. Future designs should utilize an average operating environment (AOE). By determining an AOE, we can depict a more realistic operating scenario and obtain a more accurate prediction of performance. The AOE is the sum of the per- centages of total operating time that the ship is expected to operate in each environment. For example, the AOE for a particular design might be:

AOE = 40% Northern North Atlantic + 30% Caribbean + 30% Eastern Mediterranean

The information needed to determine the AOE should be stated in the TLR or other OPNAV documentation. The AOE should be determined as early in the design process as possible and prior to any stack gas dispersion studies. Those environments which are com- ponents of the AOE are hereby designated as “significant” environments.

IDENTIFY THE STACK PERFORMANCE ELEMENTS

The second step of the prediction methodology involves identification of the stack performance elements. The following four stack performance elements were examined during the design of a recent surface combatant:


Topside Electronic Equipment

Excessive heat resulting from the stack gasses may damage temperature-sensitive topside electronic equipment. Each specific piece of electronic equipment that may be adversely affected should be treated as a sepa- rate SPE.

Turbine Intakes

Stack gasses may be drawn into the turbine intakes resulting in reduced engine efficiency and reduced engine reliability.

Deck Hazard

Gas sweepdown on the deck may pose a health hazard and prevent manning of the deck while the ship is underway.

Flight Operations

Stack gasses may interfere with helicopter operations. High temperature gas causes loss of lift on the helicopter blades, loss of engine power, and degraded con- trol during the critical moments before a landing.

ESTABLISH CRITERIA FOR EACH STACK PERFORMANCE ELEMENT

The lack of definite criteria for several stack performance elements hindered the stack performance prediction of a recent surface combatant design. Reference [5 ] states that “An accepted threshold for stack design is that the stack perform satisfactorily under those conditions which will occur 95% of the time. That is to say that the temperature and trajectories may exceed the design thresholds up to 5 % of the time without undue risk or unacceptable detrimental ship impacts.” At various points in the design process, threshold temperatures for some stack performance elements were uncertain. At other times, the “95% satisfactory performance” re- quirement was questioned. In future designs, detailed criteria for each SPE should be established by the cognizant code or organization responsible for that SPE prior to any stack performance analysis. Establishing criteria as early as possible in the design process will reduce any bias or claim of bias concerning the satisfactoryhnsat- isfactory performance of any particular SPE that might occur later in the design process if detailed criteria were not firmly established.

DETERMINE RELATIVE IMPORTANCE OF EACH STACK PERFORMANCE ELEMENT

The stack performance elements are prioritized and assigned values of relative importance (RI) based on the degree to which unsatisfactory performance of each SPE will adversely impact the ship. The most important SPE is assigned a RI of 10.0. The other stack performance elements are then assigned a value less than 10.0


based on their relative importance to the most important SPE . DETERMINE ALTERNATE STACK CONFIGURATIONS THAT SHOULD BE EXAMINED

Alternate stack configurations to be examined should be determined. The alternate configurations may represent combinations of various stack casing shapes, stack casing heights, or heights of the pipes protruding above the stack casings. For example, rectangular, oval, or trapezoidal casing plan form shapes can be considered.

PERFORM PLUME TRAJECTORY AND TEMPERATURE CALCULATIONS

Presently, the hull form and hydrodynamics performance division uses the computer program PLUME to estimate the probability of temperature sensitive devices exceeding their critical temperature because of hot stack gasses. The methodology used by the program is as follows:

1) Calculate the plume trajectory based on stack gas

2) Calculate the maximum plume radius based on the

3) Calculate the temperature at a given radius.

velocity.

vertical height of the plume.

These are the primary steps. Combined with enviro-n- mental statistics and ship speed probabilities, the probability of exceeding a given temperature is determined. The series of equations used by PLUME are presented in reference [6]. For each SPE the established criteria are compared to the PLUME-generated temperature predictions.

DETERMINE IF MODEL TESTING IS NEEDED

Based on the previously determined criteria and the results of the plume trajectory and temperature calculations, the hydrodynamics task leader (TL) determines if model testing is needed. Model testing should be performed if there is any question as to the certainty of the calculated plume trajectory or if the criteria of any SPE

DYE T A N K

W A T E R P U M P

S U P P O R T I N G P I P E

W A T E R S U R F A C E FROM T U R B I N E I N T A K E S G R D U N D B O A R D

I C I R C U L A T I N G W A T E R 1

S M O K E F L O W P A l T E R N IDYE S O L U T l O N l

gure 6. Schematic Setup for the Stack Gas Dispersion [periments at DTNSRDC

'I * I RECORDING OF ALL JUDGEMENTS IN A COLORCOOEO

'PERFORMANCE CHART"

SIGNIFICANT ENVlRDNMENTS

FOR 21 SECTORS IN EACH OF THE SIGNIFICANT ENWRMIMENTS. (THIS

DIVIDES THE oo SECTOR INTO 15" mm

OFLAWING OF "CUMUUTIVE PROBABIUN CURVES' FOR EACH SECTOR IN EACH

SIGNIFICANT ENVIRONMENT.

PRODUCTION OF "PROBABILITY TABLE" FOR EACH SIGNIFICANT ENVIRONMENT :

\L

FOR EACH SIGNIFICANT ENVIRONMENT

PRODUCTION OF "CONFIGUFLATION TABLE' FOR "AVERAGE OPERATING ENVIRONMENT'

Figure 7. Flowchart of Model Test Result Analysis

involves a percentage of operating time at which the SPE will experience stack gas impingement.

PLAN MODEL TESTING

Model testing should be planned to test for all stack performance elements that are under question at this point. Model testing should be carefully planned to in- sure that all conditions for which marginally unsatisfactory or absolutely unsatisfactory performance may occur will be tested. Performance charts from previous model tests of the most similar type model should be examined. If the highest velocity ratio tested in a sector from the previous performance chart showed marginally unsatisfactory performance, the new tests should include testing at a velocity ratio of 0.25 higher in that particular sector. If the highest velocity ratio tested in a sector from the previous performance chart showed absolutely unsatisfactory performance, the new tests should include testing at velocity ratios of 0.25 and 0.50 higher in that particular sector. Testing should occur at the following relative wind headings (sectors): 0 degrees, 15 degrees port, 15 degrees starboard, 45 degrees port, 45 degrees starboard, 90 degrees port, 90 degrees starboard, 135 degrees port, 135 degrees starboard, and 180 degrees.

PERFORM MODEL TESTS

Model testing is performed with the model attached upside down beneath a groundboard, submerged to a depth of approximately one foot in the circulating water channel (CWC) at David Taylor Naval Ship Research & Deve lopmen t Cen te r , Ca rde rock , Maryland



(DTNSRDC) (Figure 6). The water in the channel is cir- culated at 3 knots to represent the relative wind. Dif- ferent relative wind headings are simulated by rotating the model and groundboard in the various directions tested. Blue dyed water is pumped through the stacks to simulate the stack gasses. The flow-in through the turbine intakes is simulated. The flow rate through the stacks is adjusted to simulate various velocity ratios [4]. The tests are filmed and photographed to record the dispersion of the dye at the various conditions tested. These visual records are the only results obtained from model testing.

PERFORM ANALYSIS OF MODEL TEST RESULTS

Figure 7 illustrates the procedure used in this analysis. A two-sided approach is used to analyze the model test results. A qualitative approach (parts 1-3) is used to judge the performance of the stack configurations in the model tests. A quantitative approach (parts 4-7) utilizes the computer program PROVBR to produce the probabilities of encountering the various relative wind headings and velocity ratios in the several environments. The probability (percentage of time) of unsatisfactory performance is determined by combining the results from the qualitative approach and the quantitative approach. The combination of the two approaches occurs in parts 8-9. This procedure should be used to evaluate test results of all main stack configurations as well as any other stack configurations (such as SSGTG stacks) that are model tested. Each part of the procedure is described as follows:

V h a l Observation of Model Test Film and Photos for a Particular Condition

A panel shall observe all provided views of the model test still photographs and film for each condition. The hydrodynamics task leader should select the panel which should consist of several (at least three) engineers in- cluding the hydrodynamics TL and those engineers rep- resenting offices having cognizance over the various stack performance elements.

Judgment of Performance for Each Stack Performance Element for the Condition

For those stack performance elements whose criteria depend on the fraction of operating time at which stack gas impingement occurs, this methodology categorizes performance into four levels: absolutely satisfactory, marginally satisfactory, marginally unsatisfactory, and absolutely unsatisfactory. This four-level descriptive rating system was originally developed to reflect the several gradations of impingement that are realistically visi- ble during observation of model test results (as described below). It may in fact be preferable (and simpler) to use only three levels: satisfactory, marginal, and unsatisfactory. While this approach would obviously change the numerical results and may change conclusions as to whether performance is ultimately acceptable


or not, the basic approach presented here would not change.

Accordingly, each condition is judged by the panel to have one of the following ratings for each SPE:

Absolutely Unsatisfactory - Obvious, constant, significant impingement on the SPE. Marginally Unsatisfactory - Intermittent impingement on the SPE. Marginally Satisfactory - No impingement on the SPE but the stack gas plume barely avoids the SPE. Absolutely Satisfactory - Obviously no impingement on the SPE and the gas plume clears the SPE by a significant distance.

A discussion among the members of the panel should take place as each condition is viewed. Each member of the panel should vote independently on the judgment of each condition for each SPE. Based on the list of the panel’s votes, the hydrodynamics TL makes the final judgment on each condition for each SPE.

Recording of All Judgments in a Color- Coded ‘ ‘Performance Chart ”

Performance charts are shown in figure 8. A performance chart is produced for each SPE for each configuration tested. The relative wind heading sectors are shown on the horizontal axis. The velocity ratios are shown on the vertical axis. The judgments for each condition are entered at their appropriate positions in the chart. Color-coding enhances readability. Conditions of “No Data Available” should be designated in the chart.

DECK HAZARD

PERFORMANCE CHART FOR BASELINE STACKS 1

TURBINE INTAKES

PERFORMANCE CHART FOR BASELINE STACKS

KEY: - - - ABSOLUTELY UNSATISFACTORY lAUl

MARGINALLY UNSATISFACTORY I M U I MARGINALLY SATISFACTORY IMSI El ABSOLUTELY SATISFACTORY (AS1

Figure 8. Performance Charts


If model tests are properly planned, data (photographs) should be available for all conditions where unsatisfactory performance may exist. However, if no data is available, judgment ratings must be estimated for these conditions. Rules for estimation are as follows:

For “No Data Available” conditions with VR less than the Minimum Tested VR for a given sector:

If VR = 0.25 less than minimum tested VR, then estimated rating is two ratings lower than that of minimum tested VR. If VR = 0.50 less than minimum tested VR, then estimated rating is three ratings lower than that of minimum tested VR.

Note: Lowest possible rating is absolutely unsatisfactory. For “No Data Available” conditions with VR greater than the maximum tested VR for a given sector:

If VR = 0.25 greater than maximum tested VR, then estimated rating is same as that of maximum tested VR.

If VR = 0.50 greater than maximum tested VR, then estimated rating is one rating higher than that of maximum tested VR.

If VR = 0.75 greater than maximum tested VR, then estimated rating is two ratings higher than that of maximum tested VR. If VR = 1.00 greater than maximum tested VR, then estimated rating is three ratings higher than that of maximum tested VR.

Note: Highest possible rating is absolutely satisfactory.

Running of PROBVR Program for 8 Sectors in each of the Significant Environments

The hull form and hydrodynamics performance division uses the computer program PROBVR to predict probabilities of encountering specific relative wind heading sectors and velocity ratios in specific geograph- ical environments. One run is made for each significant environment. PROBVR documentation is presented in reference [6] . Inputs and outputs are briefly described below:

PROBVR INPUTS

1) Environmental Site: 1-9 (one environment for each run.) The wind-speed profile for the selected environment is used in the program’s probability calculations.

8 (0 degrees, 45 degrees port, 45 degrees starboard, 90 degrees port, 90 degrees starboard, 135 degrees port, 135 degrees starboard, 180 degrees.) These sectors are shown in figure 9.

This is the curve of stack exit velocity versus ship speed.

This is the ship’s peacetime operational speed profile.

2) Number of Sectors:

3) Propulsion Profile:

4) Speed Profile:

270

180

Figure 9. Sector Diagram

These inputs are used in the program’s probability calculations which produce the PROBVR output.

PROBVR OUTPUTS

1) Probabilities of being in each specific sector for travel in the specific environment.

2) Probabilities of having velocity ratios less than or equal t o certain values (0.5, 1.0, 1.5, 2.0, . . .) for each specific sector for travel in the specific environment.

Running of PROBVR for Twenty-Four Sectors in each Significant Environment

PROBVR is run again for each significant environment. Twenty-four sectors are input this time. This divides the 0 degree sector into three sectors of 0 degrees, 15 degrees port, and 15 degrees starboard. The area about the ship is broken into the same sectors that are model tested. PROBVR should be refined in the future to allow a more user-friendly method of mapping out the sectors about the ship.

Drawing of “Cumulative Probability Curves’’ for each Sector in each Significant Environment

Cumulative probability curves are drawn from the PROBVR output. An example curve is shown in Figure 10. Velocity ratio is plotted on the x-axis. Cumulative probability (the probability of having a velocity ratio less than or equal to a specified value) is plotted on the y-axis. A curve is generated for each sector. A different set of curves is produced for each environment. The curves are ,used to find cumulative probabilities for velocity ratios between those listed in the PROBVR output.



Production of the “Probability Table” for each Significant Environment

Probability tables are shown in Figure 11. The probabilities PA, PB, and Pc are entered in the appropriate positions in the table. PA values are generated in the PROBVR output. PB values are taken off the cumulative probability curves. PC values are the products of PA and PB values. The three probabilities are described as follows:

PA = The first number in each column. = The probability (in percent) of heading in that

specific sector if the ship is travelling in that specific environment.

PB = The number in the top left-hand corner box of each condition position.

= The probability (in percent) of having a velocity ratio less than or equal to the value in the left- most column of the same row if the ship is heading in that specific sector and travelling in that specific environment.

PC = The bottom number in each condition position.

= The probability (in percent) of having a velocity ratio less than or equal to the value in the left- most column of the same row and heading in that specific sector if the ship is travelling in that specific environment.

= P A x PB

ENVIRONMENT # 2: NORTHERN NORTH ATLANTIC

R-“I // m.7 46.9

58.5

0 0.5 0.75 1.0 1.25 1.5 2.0 2.5 3.0 VELOCITY RATIO, Vr

Figure 10. Example Cumulative Probability Curve


Figure 11. Probability Tables

Production of the “Configuration Table” for each Significant Environment

This is the first step in combining the qualitative and quantitative approaches of the analysis. A configuration table is generated for each SPE in each significant environment. Example configuration tables are shown in Figure 12. These tables show the probability of absolutely unsatisfactory and marginally unsatisfactory performance for each configuration in the environment. These two values are calculated for each SPE by the following methods:

1) Superimposing the performance chart of a particular configuration onto the probability table of a particular environment.

2) Looking at each sector in the performance chart and noting the highest velocity ratios at which absolutely unsatisfactory and marginally unsatisfactory performance occur.

3) Looking at the corresponding condition on the probability table and noting the probabilities of absolutely unsatisfactory and marginally unsatisfactory performance.

4) Summing the probabilities of absolutely unsatisfactory performance for all ten sectors gives the probability of absolutely unsatisfactory performance for the environment.

5) Summing the probabilities of marginally unsatisfactory performance for all ten sectors gives the probability of marginally unsatisfactory performance for the environment.


PREPARE CONCLUSIONS AND RECOMMENDATIONS

DECK HAZARD

TURBINE INTAKES

~ ~

I CONFIGURATION TABLE FOR ENVIRONMENT #2: NORTHERN NORTH ATLANTIC

3.6% 10.2%

1.7% 2.7% i

MARGINALLY I ABSOLUTELY I PE:!zGFi:cE I UNSATISFACTORY UNSATISFACTORY I STACK

CONFIGURATION TABLE FOR ENVIRONMENT #8: THE CARIBBEAN

1

DECK HAZARD

TURBINE INTAKES

ABSOLUTELY MARGINALLY STACK

1.7% 7.0%

0.7% 1.1%

TURBINE INTAKES 0.1% 0.1% I I

I CONFIGURATION TABLE FOR I THE AVERAGE OPERATING ENVIRONMENT

ABSOLUTELY MARGINALLY STACK

PE:[z~~i~cE UNSATISFACTORY UNSATISFACTORY

Figure 12. Example Configuration Tables

Production of the “Configuration Table” for the “Average Operating Environment”

A configuration table for the average operating environment is produced for each SPE. These tables are shown in Figure 12. These tables show the probability of absolutely unsatisfactory and marginally unsatisfactory performance the ship will experience in actual opera- tion. These values are generated by the following equations:

Pau (A.O.E.) = Pau(#l) F(#l) + Pau(#2) F(#2) + . . . + Pau(#9) F(#9)

Pau (A.O.E.) = Pmu(#l) F(#l) + Pmu(#2) F(#2) + . . . + Pmu(#9) F(#9)

Pau (A.O.E.) = Probability of absolutely unsatisfactory performance in the average operating environment

Pau (#x) = Probability of absolutely unsatisfactory performance in environment #x where x = 1-9.

Pmu (A.O.E.) = Probability of marginally unsatisfactory performance in the average operating environment

Pmu (#x) = Probability of marginally unsatisfactory performance in environment #x where x = 1-9.

F(#x) = Fraction of total operating time the ship will operate in environment #x where x = 1-9.

Note: F(#x) = 0 for all non-significant environments.

The final step of the methodology involves examina- tion of the results from the previous steps. The acceptability or unacceptability of each configuration studied is determined by the predicted probabilities of unsatisfactory performance, performance criteria, and relative importance of each SPE. Conclusions and recornmen- dations as to the acceptability of each stack configuration design are prepared.

FUTURE DIRECTIONS IN STACK DESIGN TECHNOLOGY

The following activities should be implemented to further advance stack design technology:

IMPLEMENTATION OF A STANDARDIZED METHODOLOGY TO PREDICT STACK PERFORMANCE

The methodology presented in this paper should be used in future design work. A computer program should be designed to perform parts 4-9 of the model test result analysis (Figure 7).

ADDITIONAL RESEARCH AND DEVELOPMENT

Although the present computer programs used in stack design are quite advanced, there is still room for improvement. It is presently believed that the computer program PLUME may tend to overpredict temperatures because of an inaccuracy in the plume radius calcula- tion. A finite-difference method to predict temperatures for single-stack exhaust is currently being developed and is expected to be fully operational by FY 85. Studies to develop a finite-difference analysis for multistack temperature prediction are underway [7].

Additional R&D is also needed in the area of stack casing shape design. A recent surface combatant design features non-streamlined trapezoidal shaped stack casings. Model test results show that these trapezoidal casings do not perform worse than an alternate casing design with rounded corners at corresponding stack heights. This result amplifies the long-known fact that height is by far the most important factor in designing for good stack performance. However, it should be noted that due to the small radius (six feet) of the corners on the rounded casings (which was even further diminished on the 1/94th scale model), the full effect of this difference in casing shape might not have been detectable in the model tests. In any case, trapezoidal stack casings are well worth further investigation because of their in- herent advantages of decreased radar cross-section and decreased construction costs.

DEVELOPMENT OF DETAILED CRITERIA FOR THE VARIOUS STACK PERFORMANCE ELEMENTS

Efforts to develop detailed criteria for the various stack performance elements should be started. The cognizant codes and organizations responsible for a partic-



USE “ S T I U DESIGN TECHNOLOGY FOR NAVAL AND MERCHANl W l K ’

ular SPE should determine the amount of stack gas impingement or ingestion and/or resulting temperatures that the SPE can tolerate. This effort may require R&D work.

CONCLUSION

The ideas presented in this paper stem from lessons learned during a recent stack design experience. The key to future successful stack design will be the establish- ment of the “ground rules” as early as possible in the design process. This will require a coordinated team effort between the hull form and hydrodynamics performance division, design management, the design team, and the various cognizant codes/organizations responsible for the stack performance elements. We must ask ourselves and answer some of the questions the original stack designers may have pondered, “What are the performance requirements of the stacks?” and “How much unsatisfactory performance are we willing to ac- cept in order to meet other constraints?” Thus, the “ground rules” for design can be established by the following actions which are steps in this proposed standardized methodology to predict stack performance:

.1) Identify Stack Performance Elements 2) Establish Criteria for Each Stack Performance Ele-

3) Determine Relative Importance of Each Stack Per-

4) Determine the Average Operating Environment

ment

formance Element

Figure 13 shows the role this methodology should play in the stack design process.

This paper deals with a very specific technical issue, namely stack performance. However, the principle behind this work is universal to the fields of engineering as well as management. We recognize a problem that will need to be dealt with in the future and propose a way to deal with that problem. Unless all new Navy ships are to be nuclear-powered, we will be designing stacks for quite some time into the future. By taking ac- tion now, the Navy will be better able to deal with the problem of stack performance when it occurs in future designs. Perhaps this point was best expressed by an innovative NAVSEA project officer when he recently told the naval engineering community that, “If you have the vision, only you can make it happen! [S ]”

REFERENCES

[l] Baham, Gary J. and Donald McCallum, “Stack Design Technology for Naval and Merchant Ships,” SNAME Transactions, Vol. 85, 1977, pp. 324-349.

[2] “DDG-47 Class Stack Redesign Study,” NAVSEC Report No. 6136-77-14, March, 1977.

[3] Ower, E. and C.H. Burge, “Funnel Design and Smoke Abatement,” Institute of Marine Engineers, November, 1950.

[4] “Stack Gas Dispersion Experiments on a Proposed Top- sides Arrangement of DDG-51,” DTNSRDC Report No. SPD-1042-01, July, 1982.


[5] “DDG-51: Preliminary Design Stack Gas Study,” NAVSEA Report 55W3-82-17, December, 1982.

[6] “Low ProfileAntegrated Stack Design,” NAVSEA Report No. 55W3-82-14, November 1982.

[7] Sundaram, T.R., “A Theoretical Model for the Prediction of the Behavior of Ship-Stack Plumes,” T.S. Associates, Incorporated, Technical Report 5841 1-4, December, 1984.

[8] Munger, Cdr. C.E., “Present and Future Concepts in Surface Ships,” ASNE Presentation, San Diego, Calif., July, 1984.

APPENDIX A

Example Of The Method To Predict Stack Performance For The Super

Surface Combatant SSC-007

This Appendix presents an example of the proposed method to predict stack performance. Each step in the methodology is briefly explained. The SSC-007 and SSC-003 are mythical ship designs used only for illustra- tive purposes.

I. DETERMINE THE “AVERAGE OPERATING ENVIRONMENT”

It was determined from operations information that the “average operating environment” could be expressed as:

AOE = 40% Northern North Atlantic + 60% Caribbean

11. IDENTIFY THE STACK PERFORMANCE ELEMENTS

The following five stack performance elements were identified:

START DESIGN PROCESS

4.J E S T M U W DESIGN “GRWND RULES’

(DETERMINE THE PERMRMANCE WE ARE DESIGNING FOR)

USE STEPS 1 . N OF THE MEIHW TO PREDICT SIACK PERFORMANCE

I

\1. I I PREDICT PERFORMANCE OF STACK DESIGN 1 USE STEPS V . XI OF THE

METIUID TO mEMcT STACK PERFORMANCE I

ieure 13. Role of the Methodolow to Predict Stack Perfor-

S E E P A STACK CDNFIGURITION DESIGN

ANY NEW DESIGN

-----&3 END DESIGN PROCESS

mince in the Stack Design Process’


1) Gas Turbine Air Intake 2) Helicopter Operations 3) Deck Hazards 4) Cosmic Electronic Warfare System (CEWS) 5) Radar Antenna XYZ

VI. PERFORM PLUME TRAJECTORY AND TEMPERATURE CALCULATIONS

The computer program PLUME was used to obtain the following temperature predictions:

111. ESTABLISH CRITERIA FOR EACH STACK PERFORMANCE ELEMENT

The following criteria were established by the cognizant code or organization responsible for each SPE.

1) Gas Turbine Air Intake Stack gasses should be drawn into the intake no more than 10% of the total operating time.

The temperature on the helicopter should not exceed 170 degrees F.

3) Deck Hazards A) Gasses of 180 degrees F or greater should never

be present on the main deck or exposed areas of the 01 level.

B) Gasses of absolutely unsatisfactory impingement shall not occur on the ship’s decks more than 20% of the total operating time.

4 ) Cosmic Electronic Warfare System (CE WS)

2) Helicopter Operations

A) The temperature on the system should never exceed 265 degrees F.

B) The temperature on the system should exceed 200 degrees F no more than 2% of the total operating time.

C) The temperature on the system should exceed 160 degrees F no more than 5% of the total operating time.

5) Radar Antenna XYZ The temperature on the antenna should never exceed 235 degrees F.

Iv. DETERMINE RELATIVE IMPORTANCE OF EACH STACK PERFORMANCE ELEMENT

The design management prioritized and assigned values of relative importance to the stack performance elements as follows:

Priority SPE Relative Importance

1 CEWS 10.0 2 Helicopter Operations 7.0 3 Radar Antenna XYZ 6.0 4 Gas Turbine Air Intake 4.0 5 Deck Hazards 2.5

V. DETERMINE ALTERNATE STACK CONFIGURATIONS THAT SHOULD BE EXAMINED

It was determined that the following two configurations should be examined:

1) Current twin-stack baseline configuration 2) Current baseline configuration with pipes and stack

casings raised five feet (alternate configuration)

Gas Turbine Air Intake Temperatures were not predicted because criterion is not temperature dependent. Helicopter Operations The probability of exceeding 170 degrees F temperature on the helicopter is 0 for both the baseline configuration and the alternate configuration. (Maxi- mum resulting temperatures are 153 degrees F and 141 degrees F respectively.) Thus the criterion for helicopter operations is met for both configurations. Deck Hazards Baseline Configuration:

Probability of exceeding 180 degrees F on main deck = 0. Probability of exceeding 180 degrees F on 01 level = 0.

Probability of exceeding 180 degrees F on main deck = 0. Probability of exceeding 180 degrees F on 01 level = 0.

Thus both configurations meet the temperature dependent criterion A. CE WS Baseline Configuration:

Alternate Configuration:

Probability of exceeding 265 degrees F = 0.0% Probability of exceeding 200 degrees F = 0.8% Probability of exceeding 160 degrees F = 6.7%

Probability of exceeding 265 degrees F = 0.0% Probability of exceeding 200 degrees F = 0.0% Probability of exceeding 160 degrees F = 1.3%

The baseline configuration meets criteria A and B only.

The alternate configuration meets all criteria (A, B, and C.)

Radar Antenna XYZ Baseline Configuration:

Probability of exceeding 235 degrees F = 1.2% Alternate Configuration:

Probability of exceeding 235 degrees F = 0.0% The alternate configuration meets the criterion and the baseline configuration does not meet the criterion.

Alternate Configuration:

VII. DETERMJNE IF MODEL TESTING IS NEEDED

It was determined that model testing was needed because:

1) There is uncertainty as to the plume trajectory in the vicinity of the turbine intake.

2) The criteria for deck hazards is dependent on the percentage of operating time that any of the ship’s decks will experience absolutely unsatisfactory impingement.

VIII. PLAN MODEL TESTING

Model tests were planned to determine at which con-

45

ditions:

Naval Engineers Journal, September 1986


1) Absolutely unsatisfactory impingement of gasses

2) Ingestion of gasses into the turbine intake will occur. upon the ship’s decks will occur.

The performance charts from the SSC-003 contract design model tests were used as a basis for determining which conditions would be tested. The maximum tested Vr in the SSC-003 tests was 2.0. Due to absolutely unsatisfactory performance at 15 degrees, 45 degrees, 135 degrees, and 180 degrees in the SSC-003 deck hazard performance chart, it was determined that the SSC-007 tests should include velocity ratios of 2.5 and 3.0.

IX. PERFORM MODEL TESTS

Model tests were performed in the circulating water channel for both configurations. The conditions tested are shown in Figure 8. Film and still photos were obtained for each condition tested for both configurations.

x. PERFORM ANALYSIS OF MODEL TEST RESULTS

Parts 1-3: The model test film and photos were ob- served. The judgments of performance for deck hazards and turbine intakes were recorded in performance charts for both configurations. The performance charts for the baseline configuration are shown in Figure 8.

Parts 4-6: PROBVR was run for both 8 and 24 sectors in both the Northern North Atlantic and Caribbean environments. All cumulative probability curves were generated. The cumulative probability curve for the 180 degrees sector in the Northern North Atlantic is shown in Figure 10.

Part 7: Probability tables were generated for both significant environments (see Figure 1 1 ) .

Part 8: Configuration tables were produced for each configuration in each significant environment. Configu- ration tables for the baseline configuration are shown in Figure 12.

Part 9: The configuration table for the average operating environment was produced for each configuration. The configuration table for the baseline configuration is shown in Figure 12. All values in the configuration table for the alternate configuration were lower than the corresponding value in the baseline configuration.

From the model test result analysis, it was concluded that both configurations meet the criterion for the turbine intake and criterion B for deck hazards.

XI. PREPARE CONCLUSIONS AND RECOMMENDATIONS

From the temperature predictions and model test result analysis, it was concluded that:

The alternate configuration meets all criteria for all stack performance elements. The baseline configuration meets all criteria for all stack performance elements except the criterion for the radar antenna XYZ and criterion C of the CEWS.

Based on the relative importance of 10.0 assigned to the CEWS and 6.0 assigned to radar antenna XYZ it was recommended that:

1) The baseline configuration is unacceptable. 2) The alternate configuration (with pipes and casings

raised five feet) is acceptable.

46 Naval Engineers Journal, Septembery 1986

a method to predict stack performance

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