Trans. Tianjin Univ. 2012, 18: 231-235

DOI 10.1007/s12209-012-1736-3

Accepted date: 2011-11-25. *Supported by National Key Technologies R&D Program in the 12th Five-Year Plan of China(No. 2011BAJ08B09). HUANG Xiaoqing, born in 1984, female, doctorate student. Correspondence to HUANG Xiaoqing, E-mail: hxq-101@163.com.

Experimental Study on Spray Cooling Performance of Pressure Atomizing Nozzle*

HUANG Xiaoqing (黄晓庆)，ZHANG Xu (张 旭)

(Institute of HVAC & Gas, Tongji University, Shanghai 201804, China)

© Tianjin University and Springer-Verlag Berlin Heidelberg 2012

Abstract：Aiming at the problem of air-cooled condenser output limit, a spray humidification system was presented to reduce the inlet air temperature. The pressure atomizing nozzle TF8 was chosen for inlet air spray cooling, and the spray cooling experiment with different layouts of nozzles were conducted. Through heat and mass transfer analysis,the cooling effect fitting correlation was acquired with evaporative cooling being the major cooling mechanism. Theexperimental results under different nozzle layouts show that when the product of dry ball and wet ball temperature difference and spray rate is smaller than 75,℃·m3/h, opening the TF8 nozzles in row 1 and row 2 (row distance is 500,mm) has better cooling effect than those in row 1 and row 3 (row distance is 1 000 mm), while when the product is larger than 75,℃·m3/h, opening the TF8 nozzles in row 1 and row 3 is superior in cooling effect to those in row 1 and row 2 . Keywords：pressure atomizing nozzle; spray cooling; fitting correlation; nozzle layout

According to China’s development planning of power industry [1], electric power stations near coal mines should be the focus in the future, but water scarcity is the greatest obstacle to the development planning. The most effective water-saving measure in electric power stations is to develop air cooling technology.

However, the application process of air cooling is significantly influenced by environmental temperature. Especially in summer, hot fluid outlet temperature of air cooler cannot meet the process requirements due to the high environmental temperature. In order to solve this problem, evaporative cooling is proposed to enhance air side heat transfer of air cooler: hybrid (dry/wet) system, deluge-type air cooler and humidification air cooler (in-cluding packing humidification and spray humidifica-tion). And among these methods, the method of inlet air spray cooling through nozzle (spray humidification air cooler), because of its low initial investment, high rate of return, relative simple system and easy modification of existing system[2-7], is chosen for study in this paper.

According to nozzle shape and spray characteristics, the type of nozzles is mainly divided as follows: pressure atomizing nozzle, rotary atomizing nozzle, two-fluid at-omizing nozzle, ultrasonic atomizing nozzle, percussive

atomizing nozzle and so on. In this study, pressure atom-izing nozzle TF8 was adopted for inlet air spray cooling, and spray cooling experiments with different layouts of nozzles were conducted. Through heat and mass transfer analysis of spray cooling, the cooling effect fitting corre-lation, with evaporative cooling being the major cooling mechanism, was acquired. Furthermore, the comparisons of different TF8 nozzle layouts were made[8-10].

1 Experimental setup

The experimental spray cooling system comprises a high-temperature flue system, a ventilation system and a spray system, as shown in Fig.1. The high-temperature flue system consists of a circulating fan, a high-

intelligence hot air stove, a wind pipe and measuring sec-tion; the ventilation system is composed of an induced draft fan, an exhaust fan, fan control device, a wind pipe, an air sampler as well as measuring section; and the spray system comprises a water tank, a high pressure water pump, nozzles, a water receiver as well as measuring sec-tion.

A rectangular box sized 2 500 mm × 1 450 mm × 800 mm was adopted as experimental section with 4 rows of

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nozzles laid on the top in row spacing of 500 mm, 725 mm spacing between nozzles in the same row, as shown in Fig.1, and the water receiver is set at the bottom. Pres-

Fig.1 Schematic of spray cooling system (unit: mm)

sure spiral nozzle TF8(Fig.2) is adopted for experimen-tal purpose with solid cone spray and 90,° spray angle as well as 1/4 inch male thread diameter. The major measur-ing parameters and instruments are listed in Tab.1. Data collection was carried out by Fluke every 10 s.

Fig.2 Schematic of nozzle structure

Tab.1 Major parameters and instruments for experiment System Measuring parameter Instrument Measuring range

Supply flue temperature High-temperature thermal resistance tem-

perature detector

Return flue temperature Thermal resistance temperature detector High-temperature flue system

Dynamic pressure of flue pipeline Pitot tube

Inlet air temperature Thermograph

Outlet air temperature Normal-temperature thermo coupler×5

Duct dynamic pressure Pitot tube

Air sampler dry ball temperature Thermometer×2 0—50,℃; 50—100,℃

Ventilation system

Air sampler wet ball temperature Thermometer 0—50,℃

Spray water temperature Thermometer 0—50,℃

Spray water flow rate Turbine flow meter LWGY-25 and

digital display meter 0—10 m3/h Spray system

Spray system pressure Pressure meter YB150 2.5 MPa, accuracy 0.4

2 Theory for experiment

The principle of heat and mass transfer is adopted for calculating the spray cooling effect of air side. And the reliability and accuracy of the experiment results are based on the accuracy of air side parameters (dry and wet ball temperatures).

Temperature difference is the driving force of heat transfer, and water vapor partial pressure difference is the driving force of mass transfer[11-13].

Assume air temperature difference is dt and air moisture content difference is dm . When contact occurs between air and water on a certain infinitesimal area dA , the sensible heat transfer can be defined as

x p bd d ( )dQ Gc t h t t A (1)

where G is the air mass flow contacting with water, kg/s;

pc is the specific heat capacity of air, J/(kg·℃); h is the

surface sensible heat transfer coefficient between air and water interface, W/(m2·℃); t and bt are the major air

temperature and the boundary layer air temperature, re-spectively, ℃ .

The moisture transfer amount can be defined as mp q qbd d ( )dW G m h p p A (2)

where mph is the moisture transfer coefficient between air

and water interface, calculated in accordance with the water vapor partial pressure difference, kg/(N·s); qp and

qbp are the water vapor partial pressure in the major air

and in the boundary layer, respectively, Pa .

The moisture transfer can be described as Eq.(3) since the water vapor partial pressure difference can be replaced by the water content difference times moisture transfer coefficient within a small temperature range:

md bd ( )dW h d d A (3)

where mdh is the moisture transfer coefficient between air

and water interface calculated in accordance with the

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water content difference, kg/(m2·s); d and bd are the

moisture content in the major air and in the boundary

layer, respectively, kg/kg.

The latent heat transfer capacity can be calculated

from the following equation: q md bd d ( )dQ r W rh d d A (4)

where r is the latent heat of vaporization when the tem-

perature reaches bt , J/kg.

Eq.(5) can be obtained since the total heat transfer

capacity z x qd d dQ Q Q . z b md bd [ ( ) ( )]dQ h t t rh d d A (5)

3 Analysis of experimental results

3.1 Analysis of heat and mass transfer processes The heat and mass transfer processes in the spray

cooling experiment are discussed as opening the TF8 nozzles in row 1 and row 2 (row distance is 500 mm)

during the first process (12 processes in total) (abbrevi-ated as row1/2 TF8 process 1st). At the beginning in the air sampler, dry ball temperature and wet ball tempera-ture at the entrance were 79.6,℃ and 34.3,℃, respec-tively, as illustrated in Fig.3. Then the dry ball tempera-ture decreased substantially with the increase of nozzle pressure (nine control pressure values, namely 0.05, 0.07, 0.10, 0.20, 0.30, 0.50, 0.70, 1.00, and 1.06 MPa, were set) and water flow after the spray system was started. The dry ball temperature approached the wet ball temperature when the nozzle pressure reached 0.50 MPa. With the increasing nozzle pressure, the decrease of the dry ball temperature was not obvious and it was gradu-ally consistent with the wet ball temperature[14,15].

Fig.3 Variation of air sampler temperature in row 1/2 TF8,process 1st

Heat and mass transfer processes in row 1/2 TF8 process 1st can be seen in Fig.4. The sensible heat trans-fer ( xQ )decreased at first with the increase of water pressure, whereas it was in balance when nozzle pressure

reached 0.50 MPa, showing a similar trend to the dry ball temperature. The latent heat transfer ( qQ ) showed an ascending trend with the increase of water content and nozzle pressure, whereas it showed a descending trend when the water supply pressure reached 0.50 MPa due to air dehumidification. And the total heat transfer ( zQ ) was in balance at first, and then decreased.

Fig.4 Heat and mass transfer in row 1/2 TF8 process 1st

Dry ball and wet ball temperature difference (t) is the driving force for air sampler dry ball temperature de-crease when evaporative cooling is the main mechanism of cooling, so air sampler wet ball temperature at the en-trance can be expressed as the cooling limit. In the 12 processes when the TF8 nozzles in row 1 and row 2 were opened, the same trend is shown (Fig.5): at first the cool-ing effect (air sampler dry ball temperature decreases) shows an ascending and uniform trend with the increase of the product of dry ball and wet ball temperature differ-ence and vaporization rate(t·R), and when t·R reaches a certain value, the cooling effect is still increasing whereas t·R will decrease due to air dehumidification. And this indicates before t·R reaches the certain value, the major driving force of air temperature decrease is evaporative cooling, and when t·R exceeds the value, the convective heat transfer between water and air is the major driving force of cooling. For this reason, the data acquired from this experiment should be fitted individu-ally.

Fig.5 Cooling effect vs t·R

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3.2 Cooling effect fitting correlation Data section with evaporative cooling as the major

driving force was fitted, and the relationship between the cooling effect dbt and the product of t and spray rate (t·L) can be expressed as[16]

db 139.40ln 361.79 825.34t t L （ ）

8.05 215.01t L ≤ ≤ (6)

where t is the dry ball and wet ball temperature differ-ence at the entrance, ℃; L is spray rate, 3m /h . In this fitting correlation, t represents the driving limit of cool-ing effect, L is related to droplet surface area, which makes this formula physically meaningful. Relevant fit-ting coefficient 2 0.91,R demonstrating the reliability of the fitting result. 3.3 Spray cooling effect comparison for different

layouts With the TF8 nozzles in row 1 and row 3 opened

during the first process (abbreviated as row 1/3 TF8 process 1st, eight control pressure values, namely, 0.05, 0.07, 0.10, 0.20, 0.30, 0.50, 0.70 and 1.00 MPa, were set), the variation of air sampler temperature as well as heat and mass transfer, as shown in Fig.6 and Fig.7, is similar to those of row 1/2 TF8 process 1st. Likewise, the relationship between the cooling effect dbt and t·L can be fitted as db 52.83ln 54.89 235.04t t L （ ）

43.98 139.07t L ≤ ≤ (7)

Fig.6 Variation of air sampler temperature in row 1/3 TF8

process 1st

Fig.7 Heat and mass transfer in row 1/3 TF8 process 1st

2 0.97,R demonstrating the reliability of the fitting re-sult.

Correlation comparison between opening the row 1/2 TF8 process 1st and opening the row 1/3 TF8 process 1st in Fig.6 shows when t·L<75,℃·m3/h, the cooling effect of opening the row 1/2 TF8 (row distance is 500 mm) is better than that of opening the row 1/3 TF8 (row distance is 1,000,mm), while when t·L>75,℃·m3/h, the cooling effect of opening the row 1/3 TF8 is better than that of opening the row 1/2 TF8.

Fig.8 Comparison of cooling effect

4 Uncertainty of experiment

The total heat transfer capacity zQ can be defined as

z ao ai a a aQ G h h V h (8)

where aoh and aih are the enthalpy of outlet air and inlet air, respectively; aV is air volume flow rate; and a is dry air density.

Therefore the maximum relative error aQE of the air

side can be written as

a a a

( )Q h VE E E E (9)

where hE is the maximum relative error of enthalpy dif-

ference;a

E is the maximum relative error of dry air den-

sity; and aVE is the maximum relative error of air volume

flow. One common experimental condition is chosen for uncertainty calculation, i.e., Δ 4.02hE % ,

a0.149E % ,

a0.78VE % , and then

a4.949QE % .

Through the accuracy analysis of air side heat trans-fer capacity, the experimental setup and method are proven to be reliable, and the results meet the need of project application.

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5 Conclusions

(1) In twelve processes that TF8 nozzles in row 1 and row 2 were opened and two processes that TF8 noz-zles in row 1 and row 3 were opened, dry ball tempera-ture reached wet ball temperature when nozzle pressure reached 0.50 MPa, whereas air sampler temperature de-crease was not obvious with the increase of nozzle pres-sure, indicating that the optimum pressure is required for cooling effect rather than increasing pressure of water spray in the spray cooling process with evaporative cool-ing being the major driving force. (2) Data section with evaporative cooling being the major driving force was fitted: the relationship between the cooling effect of opening the TF8 nozzles in row 1 and row 2 and opening those in row 1 and row 3 and t·L is acquired, the applicable range of the fitting cor-relation is offered, and the relevant fitting coefficient shows the reliability of the fitting results. (3) Fitting correlation comparison were carried out to demonstrate the spray cooling effect. The results show when t·L＜75,℃·m3/h, the cooling effect of opening the TF8 nozzles in row 1 and row 2 (row distance is 500 mm) is superior to opening the TF8 nozzles in row 1 and row 3 (row distance is 1 000 mm), while when t·L＞75,℃·m3/h, the cooling effect of opening the TF8 nozzles in row 1 and row 3 is better .

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