INFLUENCE OF RING SIZE AND LOCATION ON FLOW TOPOLOGY, HEAT TRANSFER STRUCTURE AND THERMAL EFFICIENCY IN HEAT EXCHANGER SQUARE CHANNEL PLACED WITH 30-DEGREE INCLINED SQUARE RING

This paper presents the numerical investigations (finite volume method with SIMPLE algorithm) on flow structure, heat transfer behavior and performance assessment in heat exchanger square channel placed with 30o inclined square ring (ISR). The influences of ring size and placement on flow and heat transfer characteristics are considered for laminar flow region with the Reynolds number in the range around 100 – 2000. The purpose for the insertion of the ISR in the square channel is to induce the vortex flow and also increase the turbulent mixing. The numerical result reveals that the ring size and location have effects for the changes of the flow and heat transfer behaviors in the tested section. The present of the ISR in the tested section gives the maximum heat transfer rate around 8.13 times above the smooth square channel. In addition, the optimum thermal performance at similar pumping power or TEF is around 3.10.


INTRODUCTION 1
The designs of the vortex generators have been developed by many researchers. Propose for the development is to increase the heat transfer rate and thermal performance. The installation method, production and maintenance of the vortex generator are also important factors that must be considered. The type, size, thickness, height, placement, etc., of the vortex generators in the heat exchanger have effects on flow and heat transfer. Many researchers investigated the heat transfer, pressure loss and thermal efficiency in various types of the heat exchanger installed with vortex generators such as baffle (Arani and Moradi (2019), Naqvi et al. (2019), Ameur (2019), Saravanakumar and Kumar (2019)), rib (Bahiraei et al. (2019), Bai et al. (2019), Liu et al. (2019), Li et al. (2019)), winglet (Zhai et al. (2019), Awais and Bhuiyan (2019), Dezan et al. (2019)), etc. They reported that the vortex generators can improve the heat transfer rate and thermal performance in the heat exchanger.
The investigation for the development of the vortex generator in the heat exchanger can be separated into 2 ways; numerical and experimental investigations. The experimental result has higher reliability than the numerical result. However, it must spend high cost and resource for the experimental study. The numerical investigation can save cost and resource for the study process and also describe the mechanisms in the system. The understanding on flow and heat transfer mechanisms in the tested section is an important factor for the developments of the heat exchanger and vortex generators.
In the present research, the numerical prediction is selected to study the mechanisms in the heat exchanger square channel inserted with the vortex generators. The square ring (likes baffle or thin rib) is designed to improve heat transfer rate and thermal efficiency in the heat exchanger.
The inclined configuration is applied for the square ring to reduce the pressure loss in the tested section when compared with the general structure of the ring (90 o ring). The influences of the size and position for the inclined square ring in the heat exchanger on flow topology and heat transfer behavior are investigated. The laminar flow regime with the Reynolds number around 100 -2000 is measured for the present work. The numerical results are presented in terms of flow and heat transfer structures in the tested section. The thermal efficiency in forms of Nusselt number, friction factor and thermal enhancement factor is also summarized.

HEAT EXCHANGER SQUARE CHANNEL PLACED WITH INCLINED ORIFICE
The test section is a heat exchanger square channel placed with 30 o inclined square ring (ISR) as Fig. 1   o No slip condition is applied for all sides of the channel walls and inclined rings. o The test fluid is air with the temperature around 300K (Pr = 0.707).
The thermal properties of the air are assumed to be constant at the average bulk mean temperature (between wall and inlet). o The constant temperature around 310K is applied for the channel walls, while the inclined ring is assumed to be an insulator.
o The periodic condition (Patankar et al. (1998)) is adopted for the inlet and outlet of the physical domain The numerical problem is solved by finite volume method (SIMPLE algorithm). The tested section is governed by the continuity, the Navier-Stokes equations and the energy equation as equations 1, 2 and 3, respectively. Continuity equation: (1) Momentum equation: where, Γ is the thermal diffusivity and is printed as follows: (4) The continuity and momentum equations are discretized by the power law scheme, while the energy equation is discretized by QUICK scheme. The solutions are determined to be converged when the normalized residual values are less than 10 -5 for all variables, but less than 10 -9 only for the energy equation.
The air velocity is presented in term of the Reynolds number as equation 5. The pressure drop of the tested channel is reported with the friction factor as equation 6, while the heat transfer rate is presented with the local Nusselt number and average Nusselt number as equations 7 and 8, respectively. The efficiency of the heat exchanger channel equipped with ISR is presented in form of thermal enhancement factor as equation 9.
h uD Re ρ µ = (5) Dh is hydraulic diameter of the square channel heat exchanger.
( ) The thermal enhancement factor (TEF) is defined as the ratio of the heat transfer coefficient of an augmented surface, h, to that of a smooth surface, h0, at similar pumping power.
(9) The Nu0 and f0 are the Nusselt number and friction factor for the smooth square channel, respectively.

NUMERICAL VALIDATION
The numerical validation is an important part for the numerical investigation. It must sure that the creation domain can predict flow and heat transfer configurations in the tested section with high accuracy result. Therefore, the computational domain of the square channel inserted with ISR is validated. The validations for the computational domain in the laminar regime can be separated into 2 sections; smooth channel validation and grid independence.
The comparison between the present results (smooth square channel with no ISR) and the correlation values (Cengel and Ghajar (2015)) on both Nusselt number and friction factor is done. The deviations of the Nusselt number and friction factor are around ±0.5% and ±2%, respectively. The validation of the smooth square channel can accept due to the deviations are not more than ±5%. The verification of the smooth square channel with no ISR is plotted as Fig. 2.
The grid independence check for the computational domain of the heat exchanger square channel inserted with ISR (BR = s/H = 0.15) is done by compared four models with difference grid numbers; 120000, 280000, 360000 and 420000. The numerical result shows that the grid cell in the range around 120000 -420000 gives similar Nusselt number and friction factor values. Therefore, the grid cell around 120000 is applied for all studied cases.

NUMERICAL RESULT
Numerical results; flow and heat transfer, performance evaluation, for the heat exchanger square channel inserted with ISR are reported in this part. The mechanisms in the tested channel are presented in terms of streamlines in transverse planes, temperature distributions, local Nusselt number distributions, while the performance analysis is concluded in terms of Nusselt number ratio (Nu/Nu0), friction factor ratio (f/f0) and thermal enhancement factor (TEF). The temperature distribution in transverse planes is an indicator to check the change of the thermal boundary layer on the square channel walls. In general, the blue contour (cold fluid) is found at the center of the channel, while the red contour (hot fluid) is detected near the channel walls for the smooth channel with no ISR. The installation of the ISR in the tested channel can change the heat transfer behavior. The change of the thermal boundary layer near the heat transfer surface is found. The red layer at the right corner of the plane is clearly found to be thinner when augmented the s/H, while the red layer at the left corner slightly increases. The distribution of all color layers has change when changed the s/H value. This is because the gap between channel wall and ISR, which induces the vortices, that helps a better fluid mixing. Considering at the right corner of the plane, the s/H = 0.30 performs the lowest red layer thickness, while the s/H = 0 provides the opposite behavior. Figs. 7, 8, 9, 10, 11 and 12 present the local Nusselt number distributions on the channel walls of the heat exchanger equipped with ISR at BR = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30, respectively. The channel wall number is presented as Fig. 6. In general, the present of the ISR in the heat exchanger channel can enhance heat transfer rate higher than the smooth channel in all investigated cases. The peak of heat transfer regime has change when changed the s/H value.

Flow structure and heat transfer
For BR = 0.05, the maximum of heat transfer rate for w1 and w2 is found at s/H = 0.10, while found at s/H around 0.20 -0.30, when considered at w3 and w4.
At BR = 0.10, the s/H = 0 performs the highest heat transfer rate at w1 and w2. The heat transfer rate at w3 and w4 increases when s/H > 0. The heat transfer rate for w1 and w2 decreases when enhanced s/H value.

Performance evaluation
Figs. 13a, b, c, d, e and f report the relation of the Nu/Nu0 with the Reynolds number at various s/H for the heat exchanger square channel equipped with ISR of BR = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30, respectively. As the figures, the installation of the ISR in the heating section provides higher heat transfer coefficient than the smooth channel with no ISR in all studied cases. The Nusselt number ratio is higher than 1. In almost cases, the heat transfer rate increases when augmenting the Reynolds number. In the range studies, the Nusselt number is around 1.00 -8.13 times greater than the smooth square channel for BR = 0.05 -0.30, Re = 100 -2000 and s/H = 0 -0.30. Figs. 14a, b, c, d, e and f plot the variation of the f/f0 with the Reynolds number with various s/H for BR = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30, respectively. The fitting of the ISR in the heating system brings the higher friction loss when compared with the smooth channel in all tested cases. The friction factor ratio is higher than the unity. The f/f0 enhances when augmenting the Reynolds number. In the range investigates, the f/f0 is found to be around 1.00 -20.55 for the ISR with BR = 0.05 -0.30, Re = 100 -2000 and s/H = 0 -0.30. Figs. 15a, b, c, d, e and f depict the relation of the TEF and Re for the heat exchanger square channel equipped with ISR at BR = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30, respectively. Due to the installation of the ISR in the tested section enhances both heat transfer rate and pressure loss, therefore, the thermal efficiency is selected to analyze the efficiency of the ISR. The thermal efficiency is reported in terms of thermal enhance factor or TEF, which considered at similar pumping power. Almost cases, the insertion of the ISR provides higher TEF than the smooth channel or TEF > 1. The optimum TEF is around 2.10, 2.16, 1.98, 2.41, 3.09 and 2.97, respectively, for BR = 0.05, 0.10, 015, 0.20, 0.25 and 0. The generation of the vortex flow is found when installed ISR in the heat exchanger channel. The disturbance of the thermal boundary layer is also found in the present investigation. These behaviors are reasons for the augmentations of the heat transfer rate and thermal efficiency.
The size and position of the ISR have effect for the changes of heat transfer and flow behaviors. The small vortices near the channel walls are detected when s/H > 0. The optimum s/H value may give high heat transfer rate and thermal performance. For low BR, the suggestion of the s/H is around 5 -15%, while around 20% when BR > 0.20.
In comparison, the stable of the installation for the present vortex generator are higher than the previous types ( (2014)).