Heat Transfer Characteristic on Wing Pairs Vortex Generator using 3D Simulation of Computational Fluid Dynamic

Vortex generators are addition surface that can increase heat transfer area and change the fluid flow characteristics of the working fluid to increase heat transfer coefficient. The use of vortex generators produces longitudinal vortices that can increase the heat transfer performance because of the low pressure behind vortex generators. This investigation used delta winglet vortex generator that was combined with rectangular vortex generator to Reynold numbers ranging 6.000 to 10.000. The parameters of Nusselt number, friction factor, velocity vector and temperature distribution will be evaluated.


Introduction
Heat exchanger is one of various important components in the industries. Chemical industry, power plants, food factories hospitals, and super computers are using heat exchanger for their daily operation. Heat exchanger is used to transfer heat such as cooling system in super computers, boilers in power plants, evaporators to dry food at food factories, hot and cold piping system in hospitals and separator in chemical industries. One type of heat exchangers that is used most in the operation of those industries is circular tube heat exchanger.  Reynolds number of 6.000 to 33.000. The result shows the decrease of Nusselt number with pitch row but increase with attack angle and blockage ratio [3]. Liu et al., 2018 numerically and experimentally studied heat transfer performance on circular tube heat exchanger enhanced with rectangular winglet vortex generators [4]. Reynolds number ranging from 5.000 to 17.000 was used to investigate the effect of rectangular winglet configuration results in the increase of heat transfer rate by 54% to 188% and flow resistance by 152% to 568%. The use of v-shaped configuration results in the increase of heat transfer rate by 60% to 118% and flow resistance by 141% to 644% [9].
Based on those previous studies that are reviewed above, there are still many improvements that can be done by researchers. This study focus on heat transfer enhancement using a novel type of vortex generators, parallelogram winglet vortex generator. Parallelogram winglet vortex generator is a new type of vortex generators that is inspired by combining the characteristics of delta winglet vortex generator and rectangular winglet vortex generator.
Parallelogram winglet vortex generator has the shape of delta winglet vortex generator that is combined with rectangular vortex generator. Heat transfer enhancement of circular tube heat exchanger using parallelogram winglet vortex generators was studied numerically with plain circular tube heat exchanger as the baseline. Reynolds number ranging from 6.000 to 10.000 was used in this study to identify the heat transfer performance with the variations of parallelogram vortex generators lean following the flow and lean against the flow configuration. Nusselt number, friction factor, velocity vector and temperature distribution were used in this research as evaluation parameters.

Research Methods
Three dimensional numerical method was carried out in this study to investigate heat transfer performance of circular tube heat exchanger embedded with parallelogram winglet vortex generators.

Model Description
The simulation was carried out in two different parallelogram winglet vortex generators configurations that are lean following the flow as can be seen at Figure 1, and against the flow as can be seen at Figure 2. The length of the pipe is 700 mm with vortex generators embedded inside the circular tube. As can be seen at Figure 3

Boundary Condition
Three dimensional numerical method was carried out using Reynolds number ranging from 6.000 to 10.000. The working fluid was assumed to be steady turbulent flow. Inlet temperature of the working fluid was 322.2 K. The wall and the vortex generator assumed to have the same temperature of 300 K. The working fluid used in this study was ammonia. Coupled pressure and velocity in governing equation in this study was solved using the SIMPLE algorithm. The thermal convection and the velocity was discretized by second order upwind. The residual criteria less than 10-6 was used for the energy equation and 10-4 for the other variables.

Results and Discussion
Heat transfer performance of the cases studied was evaluated using Nusselt number and thermal gradient. Figure 5 shows the Nusselt number increases with Reynolds number that is similar in the tendency of the study by Liu et al [4]. This study has overall Nusselt number higher than Liu et al [4] study due to the use of ammonia as the working fluid that is lower in thermal conductivity than water used by Liu et al study.
The Nusselt numbers in Figure 5 increase with Reynolds numbers in all cases studied. In the case of the plain circular tube the increase of Nusselt number from 6.000  Temperature gradient was also used in this study to understand the heat transfer phenomena occurred. Figure 6, Figure 7, and Figure 8 showed the thermal gradient of the plain tube, lean following the flow configuration, and lean against the flow configuration using Reynolds number 6000. The temperature gradient of Reynolds number 6000 was used to investigate the temperature distribution characteristic due to the highest increase of the Nusselt number. The plain case in Figure 6 showed that the working fluid temperature has a smooth pattern from the inlet to the outlet. It means that the working fluid has relatively low temperature distribution. The use of vortex generator increases the temperature distribution due to the increase of contact surface between the working fluid and tube wall [Liu et al., 2018]. The use of vortex generator in Figure 7 and Figure 8 showed the better temperature distribution compare with the plain case in Figure 6. The high temperature not only occurs in the middle stream but also near the tube wall. However, temperature distribution in the case of leaned against the flow configuration showed by Figure 8 is higher than the case of leaned following the flow configuration showed by Figure 7.  c. The use of leaned following the flow configuration can increase the Nusselt number ranging from 4.39% to 7.10% while the use of leaned against the flow configuration 21.04% to 25.36%.