Proceedings of the 7th International and 45th National Conference on Fluid Mechanics and Fluid Power (FMFP) December 10-12, 2018, IIT Bombay, Mumbai, India FMFP2018–PAPER NO. 563 Heat Transfer and Fluid Flow behaviours of a Square Twisted Micro-Channel Sampad Gobinda Das Department of Mechanical Engineering, Jadavpur University, Kolkata, West Bengal, India. [email protected] Suvanjan Bhattacharyya Department of Mechanical and Aeronautical Engineering, University of Pretoria, South Africa. [email protected] Arnab Banerjee Department of Mechanical Engineering, MCKV Institute of Engineering, Howrah. West Bengal. India. [email protected] Himadri Chattopadhyay Department of Mechanical Engineering, Jadavpur University, Kolkata, West Bengal, India. Email: [email protected] efficient cooling systems. Since the extensive investigation by the previous researchers illustrated that the utilization of micro-channels are allowed for raised heat flux level to be sustained, micro-channels have become a concerning topic of investigation. Successive assessments by many researchers followed, and a variety of contradictory results were published in terms of friction factor and Nusselt number characteristics in comparison to the conservative macro range ducts behaviour. Abstract The heat transfer and pressure drop analysis of twisted square micro-channel is studied numerically for Reynolds number (Re) ranging from 10 to 250. The laminar flow model is used for formulation in Ansys fluent 15.0, since the flow is laminar. The governing equations are solved with a finite-volume-based numerical method. A threedimensional non uniform grid was generated, in order to critically examine the flow and heat transfer. The effects of aspect ratio and twist ratio on the Nusselt number and friction factor are investigated. It is found that the twisted geometry of the channel evidently enhance heat transfer by generating longitudinal vortices, which strengthen flow mixing. Such small vortices also upsurge the turbulent kinetic energy (TKE) and decrease thickness of the boundary layer, which lowers local temperature nearby the target surface. Computational results show that the simulation with twisted micro-channel geometry considerably better than the case with straight microchannel with respect to heat transfer performance. Kandilkar et al. [1] reported in details about heat exchange characteristics and the flow behaviour for micro-channels and mini-channels. The device properties were also listed. Kheirabadi et al. [2] presented a comparison of different cooling technologies. The work examined liquid cooling; spray cooling, air-cooling, pool boiling, heat pipes and jet impingement. It was predicted that by the year 2020, liquid cooling would be the norm for high performance computing, whereas air-cooling will be viable for general purpose computing. Leng et al. [3] numerically experimented and proposed design improvement in micro-channel heat sink having double layers. In integrated circuits, forced liquid cooling having high-performance was experimentally investigated by Tuckerman et al. [4]. Micro-channel heat sinks with liquid flow in single phase was studied by Steinke et al. [5], to improve upon the high heat removal characteristics. The influence of geometrical parameters in cooling efficiency Keywords: Micro-channel, Twisted channel, Heat transfer enhancement, Fluid flow, Thermal performance. I.INTRODUCTION The pressure drop and heat transfer phenomena in microchannels are significant in thermal design engineering, likely, manufacturing of electronics cooling. Understanding of the effectiveness of micro-channels in the laminar through transitional to turbulent regime flow is crucial to ensure thermally effective and energy 1 momentum equations for a three dimensional, laminar flow forced convection of heat transfer are solved. was analysed by Ghani et al. [6] with varying channel shapes. Dirker et al. [7] studied laminar and transitional regime in micro-channels and evaluated corresponding heat exchange coefficient and friction factor. For sinusoidal micro-channel having rectangular crosssections, the heat exchange and flow characteristics were experimentally investigated by Sui et al. [8]. Flow and thermal characteristics, were numerically investigated by Xia et al. [9] in micro-channel heat sinks having Y-shaped bifurcations internally. Secondary flow's effect on augmentation of heat exchange was numerically investigated by Lee et al. [10] for oblique fin microchannel. For micro-channel heat sink, the heat exchange and laminar flow characteristics were investigated by Li et al. [11], for modifications with rectangular ribs and triangular cavities. III. NUMERICAL SOLUTION Grid sizes of a variety of dimensions were tested as a part of study for grid independence. Performing a rigorous grid independency test, a mesh containing 98,775 nodes and 1,99,333 elements were utilized for this current numerical analysis. Theoretical equations of laminar model were solved for 3D model consisting diameter of 0.5 mm and length of 25 mm, under the heating condition of the test fluid, air Having Prandtl Number (Pr = 0.707). The 3D geometry model was constructed using ANSYS Design Modeler15.0. The equations for discretization of control volume were deduced from these basic equations by utilizing the hybrid scheme. The numerical estimations of the flow field are executed utilizing the Semi-Implicit Method for PressureLinked Equations (SIMPLE) algorithm. It has been observed from the literature review that heat transfer by virtue of fluid flow in twisted square channel has not been reported in the past. In this paper, therefore, the laminar flow numerical heat transfer and pressure drop results of twisted square channel of different twist and aspect ratio are presented. The 3D co-ordination of grid is fabricated utilizing ANSYS Fluent 15.0. Considering the air flow through the duct with heat exchanging, the applied mathematical model is constructed of the conservation equations of energy, momentum and mass for incompressible flow for 3D. II. BOUNDARY CONDITIONS AND MATHEMATICAL MODEL The full length square twisted micro-channel as shown in Figure 1, of inlet diameter (D) with pitch (p) and length (L) is employed. Air flow was introduced at inlet of the channel and a pressure outlet condition was applied at exit. Air inlet temperature of 298 K was set in the direction of fluid flow. The temperature of wall was kept constant at 350 K throughout the experiment. At mean bulk temperature, the thermal and the physical properties of air were taken to be invariant and no-slip condition was implemented at wall. IV. RESULTS AND DISCUSSION Figures 3(a) show the important understanding of dimensionless velocity ((u = ui/V)) at Re 250 for the twisted channel with AR 0.8 and Y = 2.0 at 5% TI at inlet. It can be witnessed that twisted geometry has solid impact over the flow field. The dimensionless thermal interaction [((T-TIN) / (TW-TIN))] is shown in Figure 3 (b). Figure displays the contour plots of temperature field in transverse planes Re = 250 for the twisted channel with AR 0.8 (AR stands for channel Aspect Ratio which is defined by height to width ration of the channel) and Y = 2.0. (‘Y’ denotes the twist ratio which is defined by the ratio of inlet diameter to the pitch of the channel). One can see from the figure that there is a change in the temperature field throughout. Figure 1: Computational Domain Steady state, incompressible flow of a Newtonian fluid with constant material properties and negligible buoyancy are assumed. The basic forms of continuity, energy and 2 Figure 2: Surface Grid Correlations established by Shah and London [16] were utilized to evaluate and validate the results of the analysis. The data found simulation of the plain micro-channel are quite close with the predicted results from the proposed correlations with little margin of error with data range of +5.2% to +6.5% and +1.8% to +2.3% for the Nusselt number and friction factor, respectively as revealed in Figure 4. (a) As established by Shah and London [26] and also presented in Figure 5, the Nusselt number (Nu) shows a specific trend that Nu rising with the rise in Reynolds number. The twisted nature in the channel aids to efficiently increase the heat transfer by distracting the boundary layer growth and also by creating local turbulence and swirl flow and thus, increasing the value of Nusselt number which can be clearly observed in Figure 5. (b) The characteristic of pressure drop in the form of friction factor (f) is presented in Figure 6. The figure represents the association between the friction penalty and Reynolds number for variety of aspect ratio and twist ratio of the square twisted micro-channel. It could be well established from Figure 6 that the friction penalty was found to follow the analogous trend, both for the plain and twisted micro-channel. The friction penalty of square twisted micro-channel is inversely proportional to Reynolds number. At a distinct Reynolds number, the microchannel with twisted nature phenomena results in greater friction factor over those of the plain micro-channel. This was due to the blockage of flow, greater contact area of the surface, the phenomenon caused by the swirl flow as well as the dynamic pressure dissipation of the fluid due to the viscosity loss nearby the micro-channel wall. Furthermore, the pressure penalty had a greater prospect to occur by the interaction of the inertia forces with pressure forces in the boundary layer. Figure 3: Dimensionless contours (a) axial velocity, and (b) temperature (a) On Figure 7, the curve of thermo-hydraulic performance parameter (PEC) with Reynolds number is shown. The thermal performance is the ratio of the dimensionless Nusselt number to dimensionless friction factor and shows the amount of the energy saved. By doing all the simulation on square twisted micro-channel and plain micro-channel it was found somewhat efficient from the energy point of view. fig. 7 shows that the channel with A.R. 1.0, with different twisted ratio provided higher thermal enhancement efficiency. (b) Figure 4: Plain channel: (a) Nu vs Re, (b) f vs Re 3 Thermo-hydraulic transport phenomena of square twisted micro-channel of various aspect ratio and twist ratio were assessed numerically. The utilization of square twisted micro-channel produces extensive augmentation of heat transfer with a substantial increase in friction factor. In general assessments, it was ascertained that the heat exchange, temperature at outlet and friction factor extensively rise at AR = 0.8, Y = 2.0. It is also evident that, the Nusselt number proportionally varies with the Reynolds number while the contrary was found for the instance of friction factor. The Nusselt number for the micro-channel with twisted geometry has a reasonable enhancement than those of the plain micro-channel values. REFERENCES Figure 5: Variation of Nusselt number with Reynolds number at different pitch and aspect ratio [1] S.G. Kandlikar, M.R. 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Wang, Characteristics of laminar flow and heat transfer in microchannel heat sink with triangular cavities and rectangular ribs, Int. J. Heat & Mass Transfer, 98 (2016), 17–28. [12] R. Shah, A. London, Laminar flow forced convection in ducts, New York: Academic Press, (1978), 196-222. Figure 6: Variation of friction factor (f) with Reynolds number at different pitch and aspect ratio Figure 7: Variation of PEC with Reynolds number at different pitch and aspect ratio V. CONCLUSION 4
