A Study on flow boiling characteristics in a minichannel and conventional channel

The flow boiling characteristics of minichannel and conventional have been evaluated for the various mass flux and heat flux. The impact of channels heat transfer analysis was carried out by mixture model (MM) of ANSYS Fluent by using water as a working fluid. The aim of this paper to find the better heat transfer in the channels at a different range of mass flow rates (0.020kg/s to 0.040 kg/s) and heat fluxes (1000 W/m2 to 2000W/m2). The analysis was explained with help of output parameters like vapour fraction, heat transfer coefficient, evaporative thin film thickness, temperature difference in channel wall side and film water temperature side, Nusselt number and friction factor. The validation was done with the help of the previous paper. It was observed that friction factor is calculated for various mass flow rate in channels, is lower from smaller diameter pipe but heat transfer is more for the case of bigger diameter. The minimum mass flow rate increases the wall surface temperature as well as high heat flux increase the channel wall temperature. The 4 mm conventional channel is better heat transfer rate compared to 3 mm minichannel.


Introduction
In recent years, the minichannel and conventional channel have been in wide use much application such as air conditioning and refrigeration, chemical industries, aerospace, electronics, desalination and distillation process etc. [1][2][3][4][5] The channel dimension classified and proposed by kandlikar and Grande [2].The channel diameter more than 3 mm is called conventional channels and 3mm ≥ D> 200 μm is called minichannels. The small diameters channels have more compact and less expensive designs but it has some limited application.
Flow boiling heat transfer has two mechanisms. One is nucleate boiling and the other one is forced convection with evaporation. The flow boiling mechanisms dependent on the mass flux and vapour quality. Particularly the flow boiling heat transfer coefficient depends on the heat flux, mass flux and vapour quality [6]While many researchers focused on flow boiling heat transfer in minichannel but the fundamental reasons for the flow boiling heat transfer mechanism, especially bubble elongation behavior and flow parameters characteristics has not explain between the minichannel and conventional channels [7][8][9].
The previous work, the three boiling model comparison done in a horizontal microchannel by Thavamani and Kumar [10]. Particular the prediction of vapour fraction and found the best model was mixture model. The estimation of vapour bubble is varying in the EM, MM, and VOF model. In the IOP Publishing doi:10.1088/1757-899X/1130/1/012032 2 present work, the working fluid as a water and to determine the effect of heat flux and mass flow rate in minichannel/conventional channel. The simulation has been investigated with different mass flux (0.020 kg/m 2 s, 0.030kg/m 2 s, 0.040kg/m 2 s) and various heat flux (1000 Wm -2 , 1500 Wm -2 , and 2000 Wm -2 ) by using the water. The second objective is to estimate the various heat transfer chacteristics such as vapour fraction, heat transfer coefficient, evaporative thin film thickness, channel film temperature-wall temperature, Nusselt number and friction factor in the channels.

Governing Equations
Generally, laminar flow occurs in micro-channel because of their dimensions is very small. The Reynolds's number value much smaller than 2.   [11].The grid independence test are conducted in the rectangular channel at the different number of nodes with the same operating condition. The entire computation was run with number no nodes 93031. Finally, no-slip boundary conditions considered in fluid wall side.

Validation
The mixture model is the best simulation model for prediction of vapour fraction and reported in the published paper [10].The validation result is shown in figure 2.The mini and conventional channel has excellent heat removal capabilities in some application. When the selection of working fluid as water, the flow rate selected as lower .The ranges of mass flux selected from 1 kg m -2 s to a maximum of 50 kg m -2 s [11].

Effect of vapour fraction for constant heat flux1500 W m -2 with different heat flux in the minichannel and conventional channel
The vapour fraction estimated at constant heat flux with a different mass flow rate in the channels. The vapour fraction gradually decreases with increases in mass flow rate. The vapour fraction increases in 3 mm minichannel compared to 4 mm of the conventional channel. Because convective latent heat of vaporization is more in the 3 mm of minichannel and frictional factor very less. It is shown in Figure 4 and Table 4.   The compact of the minichannel and conventional channel has been widely used in the various thermal application. The different type of channel used to higher heat rejection capability with the lower flow resistance. Two different widths of the channel are estimated with their heat transfer coefficient, the temperature difference between the wall and fluid, Nusselt number and friction factor by the fluent software with license version 18.1.
The heat transfer coefficient directly professional to the thermal conductivity of water and nusselt number, indirectly professional to the hydraulic diameter. The channel heat transfer coefficient depends on the channel wall temperature, water film temperature and film layer thickness. The channel water film temperature calculated from the average of inlet and outlet of channel temperature. The channel temperature difference (Tw-Tf) gradually increase and decrease of heat transfer coefficient at various mass flow rate with constant heat flux. It is shown in figure 5. Conventional channel heat transfer coefficient very high compared to 3mm minichannel heat transfer coefficient at different mass flow rate with constant heat flux because of the vapour bubble elongation very less due to frictional force. It is shown in figure 6.

Conclusions
This article presents numerical results of flow boiling heat transfer in minichannel and conventional channel using the water as the working fluids. It shows that both heat transfer and mass transfer affected by the various diameter of the channel. The grid-independent test conducted with different nodes, same time step. The vapour fraction evaluated and reported. The main concluding points of the numerical investigation in this study are the following.
• The mixture model is selected for numerical simulations in the minichannel and conventional channel. Each case of the minichannel and conventional channel estimated by different heat flux and mass flow rate at the same boundary conditions. The time iteration step was 0.25s • The vapour fraction predicted and estimated in the channel. The minichannel vapour fraction was more compared to conventional channel due to physics of bubble growth inside the channel. But the heat transfer coefficient is high in the conventional channel compared to minichannel. The evaporation film thickness was measured for the elongated bubble flow at two dimensionless distance from the bubble head.