Analysis of Helical Coil HX

ANALYSIS OF HELICAL COIL HEAT EXCHANGERS

By V.Swapna Priya

GuideR S Maurya

CONTENTS

Introduction - Outline of the project

Literature Review

Objective and Scope-Methodolgy

Experimental Setup- Schematic of coil

Experimental Details

CFD Modelling

Schematic of coil and Grid Independency

Simulation parameters and Convergence Criterion

CFD Modelling results

Validation of results

Parameters under investigation

Conclusion

OUTLINE OF THE PROJECT

Theoretical analysis of a helically coiled heat exchanger considering fluid-fluid heat transfer

Heat transfer characteristics inside a helical coil for various boundary conditions are compared

An experiment is done and the heat transfer characteristics are compared with the CFD calculation results using the CFD package FLUENT 6.2

Finally a correlation is developed to calculate inner heat transfer coefficient of the helical coil

S.No. Name Year Work Done

1 Dravid 1971 Effect of secondary flow on laminar flowNu = ( 0.65√De + 0.76) Pr 0.175

2 Yang 1994 Fully developed laminar convective heat transfer temperature gradient increased on one side of pipe wall and decreased on other.

3 Rennie & Raghavan

2006 PHOENICS 3.3, CFD package.The flow in inner tube was the limiting factor for overall heat transfer coefficient

4 Sillekins 1999 Finite difference discretization and effect of buoyancy forces on heat transfer was considered

5 Lin & Ebadian

1998 3-D Turbulent developing convective heat transfer in helical pipes. Effects of pitch, curvature ratio and Reynolds number were studied

Single Phase Flow

Two - Phase Flow

S.No. Name Year Work Done

1 Kang 2006 Heat transfer and pressure drop characteristics of HFC-134a refrigerant in a helicoidal tube

2 Berthoud & jayanti

2002 Studied the effects of coil diameter , mass flux and heat flux in helical coil

3 Guo 2002 Effects of pulsation on transient convective heat transfer characteristics of steam water two phase flow in a helical coil

4.094.0* Pr)(Re3.2Nu

LITERATURE REVIEW

OBJECTIVE AND SCOPE

Theoretical analysis of a helically coiled heat exchanger considering fluid-fluid heat transfer

Heat transfer characteristics inside a helical coil for various boundary conditions are compared

An experiment is done and the heat transfer characteristics are compared with the CFD calculation results using the CFD package FLUENT 6.2

METHODOLOGY

Numerical Simulations are done for helical coil under various parameters and coil characteristics are studied

Experiment is done and effectiveness is obtained

F- Flow ElementT- Temperature Element

EXPERIMENTAL SETUP

SCHEMATIC OF COIL

3 ½ turns helical coil

Pipe dimensions:10mm ID, 12.7mm OD

Tank dimensions: 270mm ID, 330mm OD

Pitch circle diameter: 300mm

Pitch: 30mm

DIMENSIONS OF HEAT EXCHANGER

An experiment is done on a helical coil to study the characteristics of heat transfer and hence find LMTD and effectiveness

Cold flow rate is made constant and temperatures are noted down by varying hot flow rate and air flow which is supplied externally.

The temperature values are taken after reaching steady state.

Experiments are carried out for five different flow rates through the coil and for three different values of temperature at the inlet of the helical pipe.

Using temperature values and property values, overall heat transfer coefficient is found out from various standard correlations obtained from literatures and validated

OBJECTIVE AND EXPERIMENT DETAILS

CFD MODELLING

BOUNDARY CONDITIONS

Inlet velocity (Water phase) 2.0 m/s

(Air phase) 2.0 m/s

Temperature 360K

Outlet

Temperature ( Back flow Temperature) 340Kfor both air and water phase

Wall (Mixture) Temperature 300K

A constant wall temperature of 300K was specified as the boundary condition.

Hot water at a temperature of 360K is entering the helical coil at the top and leaving at the bottom.

In the first series of analysis, the properties of water were kept constant corresponding to fluid inlet temperature and pressure(360K and 1atm)

Second analysis was done using the following temperature dependant properties

9.4631478.10031078.0051105.1)(

0294.1010879.005261.2085362.1)(

8.12270726.3011778.0055629.1)(

33158.00037524.0056028.108055.3111897.2)(

23

23

23

234

TTTeTC

TTeTeT

TTTeT

TTeTETeT

P

SCHEMATIC OF THE COIL USED FOR CFD MODELLING

INLET

OUTLET

GRID INDEPENDANCY

Structured grids created using GAMBIT 2.2

Optimum mesh chosen was 873,760 nodes, 4.516,224 faces and 1,915,833 volumes.

SIMULATION PARAMETERS AND CONVERGENCE CRITERON

Mass flow rate of cold water is kept at 0.2124 kg/s which is the same one used in experiment

Hot inlet is at 360K and cold water inlet is at 300K

The realisable k-ε model is used in this analysis

Linear discretization is used for pressure and turbulent kinetic energy and turbulent dissipation rate is used for momentum

Second order up winding is used for energy equation

A convergence criterion of 1.0e-05 is used for continuity and x,y,z velocities.

For energy equation convergence criterion is 1.0e-08 and for k and ε is

1.0e-04

CFD MODELLING RESULTS

No heat transfer

With heat transfer

The CFD simulations also extended to coils of various pitch circle diameters , tube pitches, and pipe diameters

Different cases considered arePCD 200 P30 r10 PCD 300 P45 r10 PCD 400 P30 r10PCD 300 P30 r10 PCD 300 P45 r20PCD 300 P45 r05 PCD 300 P60 r10

Velocities of water-phase and air-phase are found to be almost same As pitch is increased, which leads to higher torsional effects, the horizontal symmetry is lost.

In the beginning void fraction is highest at the inner side of the coilAlso, the effect of pitch on void fraction is negligible at the beginning of the coil

Pressure for air-phase shows local maximum towards the top of the coil. Its absolute value increases with increase in pitch.

EFFECT OF VARIOUS PARAMETERS

VALIDATION OF RESULTS

41.09112.0 Pr025.0 DeNu

PARAMETERS UNDER INVESTIGATION

The next step would be to develop a correlation applicable to all helical configurations using CFD analysis.

Also applicability of realisable k-ε model can be further investigated with staunch centrifugal and torsional effects in a helical coil.

CONCLUSION

An experimental setup is fabricated to study fluid–fluid heat transfer in a

helically coiled heat exchanger. Heat transfer characteristics of the heat

exchanger with helical coil are also studied using the CFD code FLUENT.

The CFD predictions match reasonably well with the experimental results

within experimental error limits. Based on the results a correlation was

developed to calculate the inner heat transfer coefficient of the helical coil.

REFERENCESRennie, T.J. and Raghavan, V.G.S., 2005, Experimental studies of a double-pipe helical heat exchanger. Exp Thermal Fluid Sci, 29: 919-924.

Rennie, T.J. and Raghavan, V.G.S., 2006a, Numerical studies of a double-pipe helical heat exchanger. Appl Thermal Eng, 26: 1266–1273.

Xin, R.C., Awwad, A., Dong, Z.F. and Ebadin, M.A., 1996, An investigation and comparative study of the pressure drop in air-water two-phase flow in vertical helicoidal pipes. Int J Heat Mass Transfer, 39(4): 735–743.

Mori, Y. and Nakayama., 1967, Study on forced convective heat Transfer in curved pipes (3rd report). Int J Heat Mass Transfer, 10: 681-695.

Mori, Y. and Nakayama., 1967, Study on forced convective heat Transfer in curved pipes (2nd report). Int J Heat Mass Transfer, 10: 37-59.

Jaya Kumar, J.S, Mahajani, S.M ., 2008, Experimental and CFD Estimation of heat transfer in helically coiled heat exchangers

Kang, H.J., Lin, C.X. and Ebadian, M.A., 2000, Condensation of R134a Flowing inside helicoidal pipe. Int J Heat Mass Transfer, 43: 2553–2564.

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