NUMERICAL ANALYSIS OF DOUBLE PIPE HELICAL COIL HEAT EXCHANGER

Project GroupAbhin Chandramohan 8302Athulya Shaji 8314Dheeraj Murali 8316Peeyush U 8341NUMERICAL ANALYSIS OF DOUBLE PIPE HELICAL COIL HEAT EXCHANGERGuided by Dr. M Jose PrakashIntroductionIn helically coiled tubes, the pipe curvature causes centrifugal forces to act on the flowing fluid, resulting in a secondary flow pattern perpendicular to the main axial flow.

This secondary flow pattern generally consists of two vortices, which move fluid from the inner wall of the tube across the center of the tube to the outer wall. Upon reaching the outer wall it travels back to the inner wall following the wall.

The secondary flow increases heat transfer rates as it moves fluid across the temperature gradient.Helically coiled tubes can be found in many applications including

Food processingNuclear reactorsCompact heat exchangersHeat recovery systemsChemical processing,Low value heat exchange,Medical equipment. Regenerative cooling in rocket engines

Characteristics Of Helical Coil Pitch (H)

Curvature ratio (r/Rc)

Pitch circle diameter (PCD)

Non-dimensional pitch (H/2Rc)

Helix angle ()

Dean number

OBJECTIVES OF THE PRESENT STUDY

The objective of this work is to study the heat transfer characteristics of double pipe helical heat exchanger.

Numerical investigation on helical coil heat exchanger will be conducted

Heat transfer characteristic will be studied by varying the fluid and geometrical parameters

Later it is expected to develop co relation connecting Nusselt Number, Prandtl Number and the Reynolds number

Model Used For Validation

ParameterDo (m) Di (m) d (m)R (m)Value 0.100 0.0400.0280.800Modeling : Done in AUTOCAD 2010 and exported to GAMBIT 2.4.6

Meshing: Done in GAMBIT 2.4.6 and mesh file is exported to FLUENT 6.3.26

Analysis: Done in FLUENT 6.3.26

ValidationSimulations were performed using four different mass flows in the inner tube 0.00835kg/s0.02504kg/s 0.04174kg/s0.05843 kg/s

For each inner flow rate, three trials were performed with annulus mass flow rates that were 1/2, 1, and 2 times the inner flow rate.

The total number of simulations performed was 12 ( four inner flow rates x three annulus flow rates).

Heat transfer v/s Inner mass flow rate of current analysisGraph published in Applied Thermal Engineering 26 (2006) 12661273 by Timothy J. Rennie, Vijaya G.S. RaghavanGeometry creation for AnalysisThe modeling of the heat exchanger is done using AutoCAD2010Two helically coiled concentric tubes were created one over the other to form a double pipe helical heat exchanger. Both tubes have the wall thickness of 6mm16 helical tube in models are drawn in AutoCAD2010The models were constructed by varying helix angle from 5o to 40o at an interval of 5o

Case A 10.79270.435855Case A 20.78370.8682105Case A 30.76861.294155Case A 40.74781.71205Case A 50.72122.113255Case A 60.68922.5305Case A 70.65192.868355Case A 80.60953.213405ModelRADIUS OF CURVATURE(m)PITCH(m)PITCH ANGLE()TOTAL LENGTH(m)Table.4.1Different models of helical heat exchanger with Coil length 5m

ModelRADIUS OF CURVATURE(m)PITCH(m)PITCH ANGLE()TOTAL LENGTH(m)Case B 12.98860.261553Case B 22.95440.5209103Case B 32.89780.7764153Case B 42.81911.0261203Case B 52.71891.2678253Case B 62.59811.5303Case B 72.45741.7207353Case B 82.29811.9284403Table.4.1Different models of helical heat exchanger with Coil length 3m

ANALYSIS

The models drawn using AutoCAD2010 and meshed using GAMBIT exported to FLUENT for analysis.The Boundary Conditions and Material Properties were defined in FLUENT.Two cases were analyzed to find the variation total heat transfer of fluids with geometrical parameters.

Heat exchanger fluids Inlet of the inner tubeInlet of the outer tube inletMass flow rateTemperatureMass flow rateTemperature Case 1WATER 0.055 kg/s360K0.055 kg/s290KCase2AIR 0.001078 kg/s360K0.001078 kg/s290KTable. Boundary conditionsRESULTS AND DISCUSSIONThe Effect of helix Angle on heat transfer rates of helical heat exchangers was studied and its independency from coil length, fluid in tube and different mass flow rates was proved.

Length IndependencyMaximum heat transfer was found to be at helix angle of 20o irrespective of the coil length.

13Fluid Independency Analysis have been carried out for the two fluids on all the models of length 5 m.Results were obtained for water with the flow rate 0.055kg/s and air with flow rate 0.001078 kg/s respectively.Max. heat transfer at 20o for both fluids.

water plot air plot

Flow rate Independency Analysis was carried out for the water on three models with mass flow rates 0.050 kg/s, 0.055 kg/s, 0.060 kg/s, 0.065 kg/s.The three models used were Helical coil of 5m length with helix angle 10o, 20o & 30o.Irrespective of the mass flow rates, maximum heat transfer occurred at Helix angle of 20o

Variation of Nusselt Number with Helix angleConvective heat transfer coefficient between the inner copper pipe and the inner fluid is calculated using the equation Q=hA(T2-T1)Nusselt number can be calculated as Nu=hD/kThe Nusselt no variation with helix angle was plotted for both the fluids.

Plot of Nusselt no variation with helix angle for water

Plot of Nusselt no variation with helix angle for air. Mass flow rateReynolds noNusselt NoPrandtl number.0502350.124.792.62.0552491.0425.972.62.0602717.526.822.62.0652943.9529.632.62.0703170.4230.742.62.0753396.832.392.62Formulation of heat transfer Correlation

Data obtained from the numerical experiments is used to propose a correlation for Nusselt number in terms of Reynolds number , when the helix angle is 200( helix angle corresponding to maximum heat transfer) prandtl number is 2.62 and fluid under consideration is water.

Table. Data for correlation

Correlation of the following form is appropriate. Nu = C Rem Prn The data obtained from the studies are fitted using the software MATLAB and the values of C, m and n were obtained.The proposed correlation Nu=.0588 Re.721 Pr.46 , where Pr = 2.62

ConclusionNumerical study was performed on a double pipe helical coil heat exchanger and scheme was validated using the data available in literature. The effect of helix angle on the heat transfer rates of the helical coil heat exchanger has been studied. It is found that the heat transfer increases with helix angle and then decreases. The combined effect of torsion and curvature of helical coil heat exchanger is maximum at a certain helix angle causing maximum heat transfer and was found to occur at the helix angle 200 . The angle at which maximum heat transfer occurred was found to be independent of coil length of helix, fluid passed in heat exchanger and also the mass flow rate of fluid. Finally, a correlation is proposed for the Nusselt number of water in terms of Reynolds number for helix angle corresponding to maximum heat transfer and a fixed Prandtl no of 2.62.

ReferencesHsu, C.-F., and S.V. Patankar. 1982. Analysis of laminar non-Newtonian flow and heat transfer in curved tubes. AIChE Journal, Vol. 28(4):610-616.Prabhanjan, D. G., T. J. Rennie, and G. S. V. Raghavan. 2004. Natural convection heat transfer from helical coiled tubes. International Journal of Thermal Sciences, Vol.43:359-365Rahul Kharat et al. (2009). Development of heat transfer coefficient correlation for concentric helical coil heat exchanger, International journal of Thermal Sciences 8(2009)2300-2308Sandeep. K.P et al. (2008). Heat Transfer Coefficent in Helical Heat Exchangers under turbulent flow conditions, International Journal of food engineering,vol.4,Issue 1, 2008Timothy J. Rennie, Vijaya G.S. Raghavan, Experimental studies of a double-pipe helical heat exchanger ,International journal of Thermal sciences 45 (2006) 1158-1165.Zheng. B,, C.x Lin,M.A.Edadian, Combined laminar forced convection and thermal radiation in a helical pipe,International journal of Heat and Mass Transfer(43)(2000)1067-1078

THANK YOU!