CFD Study of Single Phase Flow Distribution in Rectangular, Wavy and Offset Minichannnels with Water

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ISSN (Print): 2319-3182, Volume-1, Issue-1, 2012

107

CFD Study of Single Phase Flow Distribution in Rectangular, Wavy and Offset Minichannnels with Water

Sreebhash S Kutty & T. P Ashok Babu

Department of Mechanical Enginnering, NITK Surathkal, Mangalore E-mail : [email protected], [email protected]

Abstract – Study of minichannels is becoming more popular due to increasing developments in the field of compact heat exchangers. Three types of minichannels rectangular, wavy, offset is studied here and their flow properties are analyzed using CFD code ANSYS FLUENT for varying hydraulic diameter and mass flux. The total pressure drop is compared and the analysis is done in 2D, single phase and steady state conditions.

Keywords – CFD, Minichannels, Offset, Pressure drop, Rectangular, Wavy, Single phase

I. INTRODUCTION

The following study is carried out to understand the two dimensional velocity and pressure distribution of single phase fluid in different configurations of minichannels. Now a days compact heat exchangers are gaining immense interests in various sectors like aerospace and automobile engineering due its high area density and efficiency especially in two phase heat transfer. But before going for a two phase analysis, it is essential to have a very good understanding of single phase flow. Hence an attempt is made to conduct the CFD analysis of flow through minichannels using GAMBIT as the modeling, meshing software and ANSYS FLUENT as the solver. The fluid selected here for the study is water.

Heat transfer and pressure drop characteristics for liquid single phase flow over an array of micro pin fins in a minichannel were investigated by J.F. Tullius [3].

Six pin fin shapes – circle, square, triangle, ellipse, diamond and hexagon – were used in a staggered array and attached to the bottom heated surface of a rectangular minichannel and analyzed. Correlations of Nusselt number (Nu) with respect to Reynolds number (Re) and that of Darcy friction factor (f), in order to obtain the pressure drop, were obtained and compared to previous work. With decreasing fin width and spacing

the Nu increases; however, the increase in pressure drop is significant. Pressure drop increased significantly with a 15% increase to the unfinned channel. Selma Ben Saad [6] conducted experiments in a compact heat exchanger with two-phase inlet conditions and vertical upflow in order to study the flow behavior. The test section consists of an offset strip fin heat exchanger with a rectangular cross-section. Pressure drop has also been analyzed numerically via CFD simulations. Manglik and Bergles correlation was used to calculate the friction factor. Caney [8] studies have shown that the correlations applied for calculating single-phase pressure drop for conventional channels perform well when applied to small channels.

Single-phase pressure drop experiments were performed by A. Cavallini [1] to gain critical insight into the test section hydraulic performance. The new experimental single phase friction factors are successfully compared against predictions by Hagen–

Poiseuille, Blasius and Teplovas as well as by Churchill model over the entire range of Reynolds numbers. New experimental frictional pressure gradient data considering single-phase flow and adiabatic two-phase flow of R134a inside a single horizontal mini tube, with 0.96 mm inner diameter and with not-negligible surface roughness, was presented in the paper. Marko Matkovic [5] reported local heat transfer coefficients obtained from the measurement of the local heat flux and the direct measurement of the saturation and wall temperatures during condensation of R134a and R32 within a single circular 0.96 mm diameter minichannel.

Prior to any two-phase measurement, some tests with single phase R134a fluid flowing in the test section were carried out. The measured friction factor has found to be in good agreement with the Hagen–Poiseuille correlation in the laminar flow, while in the turbulent flow region it follows the Churcill correlation by using the measured value of roughness in the model.

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108 Krzysztof Dutkowski [4] conducted experimental investigations of pressure drop in minichannels, with use of water and air as the working fluids. The test section was made from stainless steel pipes with hydraulic diameters (D_h) of 0.55, 0.64 and 1.10 mm, respectively. The experiments were conducted in range of Re = 30 up to transition to the turbulent flow. An increase of Re number reflecting an increase of the water flow intensity, resulted in an increased flow resistance. For the minichannel diameter d = 0.55 mm, the flow resistance value was four times greater than for a minichannel with a diameter of 1.10 mm, with the same flow intensity.

Sylvain Reynau [7] measured the friction and heat transfer coefficients in 2D minichannels of 1.12 mm to 300 micrometer in thickness. The friction factor is estimated from the measured pressure drop along the whole channel. The experimental results are in good agreement with classical correlations relative to channels of conventional size. Chi-Chuan Wang [2]

conducted experiments to examine the frictional characteristics inside minichannels (D_h = 0.198–2.01 mm) with water and lubricant oil as the working fluids.

Tests were performed in both round and rectangular configurations. The test results indicated a negligible influence of viscosity on the friction factor if the hydraulic diameter is greater than 1.0 mm. In addition, the measured data can be well predicted by the conventional correlation in both laminar and turbulent flow conditions. For rectangular configuration having a gap distance of 0.1 mm, the measured friction factors for water can be roughly predicted by the conventional correlation. However, the measured friction factors for lubricant oil are slightly lower than those of predicted value. Zhe Zhang [9] used FLUENT to predict the fluid flow distribution in plate-fin heat exchangers. It is found that the flow maldistribution is very serious in the y direction of header for the conventional header used in industry. The results of flow maldistribution are presented for a plate-fin heat exchanger, which is simulated according to the configuration of the plate-fin heat exchanger currently used in industry. The numerical simulation confirms that CFD should be a suitable tool for predicting the flow distribution and optimizing the design of the plate-fin heat exchanger.

II. MODELLING AND MESHING

The modeling and meshing is carried out in GAMBIT. Five variant models of each type of minichannels are created. The five models are differentiated by varying the hydraulic diameter (D_h) from 1 – 5 mm; so a total of 15 models are created. The length of the rectangular and offset channels was taken as 28 mm; 48 mm for wavy passage to ensure flow has

developed enough to analyze a small section. Thickness of the fins were taken has 0.5 mm. A header was provided at the inlet to ensure partial uniform distribution entry of water to the inlet section of the micro channels as seen in compact heat exchangers. An enclosure was also provided to retain fluid in a confined space. The arc length of each wavy edge was taken as 2.221. The offset distance between each offset fin was taken as 0.5 mm and the length of each offset fin was taken as 1 mm. A sizing function was attached to the surfaces and the meshing was carried out using Quad – Pave scheme. Once the mesh was done the inlet and outlet boundary conditions were defined and the cell zone was defined as fluid. The model was exported as

„.msh‟ in 2D. Figure 1 – 3 represents a few models created in GAMBIT.

Fig. 1 : Rectangular channel, D_h = 1 mm

Fig. 2 : Wavy channel, D_h = 3 mm

Fig. 3 : Offset channel, D_h = 5 mm

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109 III. BOUNDARY CONDITIONS

The solver used was ANSYS FLUENT. The mesh file was imported to FLUENT. The unit was scaled to millimeters. The mesh quality was checked and was satisfactory. The energy model was enabled and laminar model was considered for analysis. Water properties were selected from materials and throughout the analysis properties are assumed constant. Wall material was set to aluminum. The cell zone conditions were set to water. The inlet boundary condition was set to mass inlet conditions. The outlet condition was set to pressure outlet conditions. In all the set of analysis the guage pressure was set to zero and the direction specification was set to normal to boundary. In mass flow inlet, the superficial/initial guage pressure was set to zero since there is no supersonic flow. The reference frame was set to absolute conditions. Mass flux was selected for mass flow specification. The mass flux was varied between 15 – 45 kg/m²s for each individual models. Wall motion was set to stationary and no slip condition was opted for shear condition. Analysis was done on pressure based and steady state condition with absolute velocity formulation. Operating pressure was at 1 bar. SIMPLE scheme was adopted for pressure velocity coupling with least squares cell based gradient; second order pressure and second order upwind momentum and Special discretization for accurate results. Then the iterations were done until the solution converged.

Table I. Water properties Water properties

Density 998.2 kg/m³ Specific heat 4182 J/kgK Thermal conductivity 0.6 W/mK

Viscosity 0.001003 kg/ms

The basic continuity equation [10] is given by

V Sm

t (1)

The momentum equation [10] is given by V

V V p g F

t (2)

2 3

T

V V V I

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= Molecular viscosity I = Unit tensor

Sm = Source term p = Static pressure

= Stress tensor

g= Gravitational body force F = External body forces V = Velocity vector

IV. RESULTS AND DISCUSSIONS

The obtained velocity profiles are shown in figure 4 – 9 for each model at varying mass flux. The comparison of pressure drop is carried out for all models in all mass flux and a conclusion of which type of fin configuration has maximum pressure drop is arrived. At 15 kg/m²s and D_h = 1 mm the velocity profile obtained for rectangular and offset minichannel configurations are shown from Fig. 4 - 5.

Fig 4 : Rectangular: Velocity Distribution

Fig. 5 : Offset: Velocity Distribution

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110 At 45 kg/m²s and D_h = 3 mm the velocity profile obtained for wavy minichannel configuration is shown in Fig. 6.

Fig. 6 : Wavy: Velocity Distribution

At 45 kg/m²s and D_h = 5 mm the velocity profile obtained for rectangular, offset and wavy minichannel configurations is shown in Fig 7-9.

Fig. 7 : Rectangular: Velocity Distribution

Fig. 8 : Wavy: Velocity Distribution

Fig. 9 : Offset: Velocity Distribution

A comparison of the percentage total pressure drop with respect to inlet pressure (% P.D w.r.t Inlet) was carried out for all the models at all the inlet boundary conditions. The obtained results are plotted as graphs.

At 15 kg/m²s the reduction in pressure drop for offset and wavy minichannels was almost linear. The pressure drop keeps on decreasing for rectangular channel and the decrease becomes prominent from D_h = 3 mm onwards.

Fig. 10 : At mass flux 15 kg/m²s

At mass flux 20 kg/m²s the decrease in pressure drop for offset channel is again linear. But pressure drop keeps on decreasing for rectangular and wavy channel and again the decrease becomes prominent from D_h = 3 mm for rectangular channel.

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111 At mass flux 25 kg/m²s the reduction in pressure drop for rectangular channel goes up to 75 % and 48 % for wavy channel.

At mass flux 30 kg/m²s the reduction in pressure drop becomes prominent from D_h = 2 mm for rectangular channel and goes up to 69 % and 47 % for wavy channel.

At mass flux 35 kg/m²s the reduction in pressure drop for rectangular channel goes up to 65 % and 45 % for wavy channel. The reduction in pressure drop for offset channel goes up to 95 %.

At mass flux 40 kg/m²s the reduction in pressure drop for rectangular channel goes up to 61 % and 42 % for wavy channel. The pressure drop for offset channel goes up to 94 %. We can see there is variations in the reduction of pressure drop of offset channels due to flow objected by fins at higher mass flux.

At mass flux 45 kg/m²s the reduction in pressure drop for rectangular channel goes up to 58 % and 41 % for wavy channel. The reduction in pressure drop for offset channel goes up to 93 %. This is configuration that gave least pressure drop at all the analyzed configurations.

Fig 16 : At mass flux 45 kg/m²s

It was observed that maximum pressure drop was found for offset fin configuration while least was for wavy fin configuration. At D_h= 1 mm the rectangular fins closely matched the pressure drop of offset fin configurations. The total pressure drop of wavy fin configurations were close to half the pressure drop of offset fin configurations. The least pressure drop was found at 45 kg/m²s and 5 mm hydraulic diameter for all three fin configurations. The maximum pressure drop for offset and rectangular fin configuration was at 1 mm hydraulic diameter and 15 kg/m²s. The maximum pressure drop for wavy fin configuration was found at 3 mm hydraulic diameter and 45 kg/m²s. As the hydraulic diameter increased the total pressure drop also kept on

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112 reducing. The maximum reduction in pressure drop was found highest for rectangular channel with increasing hydraulic diameter. As the mass flux increased the pressure drop kept on reducing and the reduction rate was more with increase in hydraulic diameter. As the diameter was increased, maximum velocities was observed in the channels closer to the inlet header. The velocity is nearly constant in all minichannels when the hydraulic diameter is 1 mm.

V. CONCLUSIONS

1. The CFD study of water flowing through rectangular, wavy and offset minichannels were analyzed at varying hydraulic diameter from 1 mm to 5 mm at varying flow rates and the velocity profiles were obtained.

2. It was observed that offset fin configuration had the maximum pressure drop while wavy minichannels showed the least pressure drop.

3. Rectangular minichannel‟s total pressure drop was close with that of offset minichannels at D_h= 1 mm.

REFERENCES

[1] Cavallini, D. Del Col, M. Matkovic and L. Rossetto,

“Frictional pressure drop during vapour–liquid flow in minichannels: Modelling and experimental evaluation,” International Journal of Heat and Fluid Flow, vol. 30, pp. 131–139, 2009

[2] Chi-Chuan Wang, Yeau-Ren Jeng, Jing-Jeng Chien and Yu-Juei Chang, “Frictional performance of highly viscous fluid in minichannels,” Applied Thermal Engineering, vol. 24, pp. 2243–2250, 2004

[3] J.F. Tullius, T.K. Tullius and Y. Bayazitoglu,

“Optimization of short micro pin fins in minichannels,”

International Journal of Heat and Mass Transfer, vol.

55, pp. 3921–3932, 2012

[4] Krzysztof Dutkowski, “Experimental investigations of Poiseuille number laminar flow of water and air in minichannels,” International Journal of Heat and Mass Transfer, vol 51, pp. 5983–5990, 2008

[5] Marko Matkovic, Alberto Cavallini, Davide Del Col and Luisa Rossetto, “Experimental study on condensation heat transfer inside a single circular Minichannel,” International Journal of Heat and Mass Transfer, vol. 52, pp. 2311–2323, 2009

[6] Selma Ben Saad, Patrice Clément, Jean-François Fourmigué, Caroline Gentric and Jean-Pierre Leclerc,

“Single phase pressure drop and two-phase distribution in an offset strip ﬁn compact heat exchanger,” Applied Thermal Engineering, pp. 1-7, 2011

[7] Sylvain Reynau, Francois Debray, Jean-Pierre Franca and Thierry Maitre, “Hydrodynamics and heat transfer in two-dimensional Minichannels,” International Journal of Heat and Mass Transfer, vol. 48, pp. 3197–

3211, 2005

[8] Wiebke Brix, Martin Ryhl Kaern and Brian Elmegaard,

“Modelling distribution of evaporating CO2 in parallel minichannels,” International Journal of Refrigeration, vol. 33, pp. 1086-1094, 2010

[9] Zhe Zhang and YanZhong Li, “CFD simulation on inlet conﬁguration of plate-ﬁn heat exchangers,”

Cryogenics, vol. 43, pp. 673–678, 2003

[10] ANSYS FLUENT Theory Guide, SAS IP, pp. 2-3, November 2011

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