NUMERICAL STUDY OF FORCED CONVECTION HEAT TRANSFER OVER CIRCULAR AND OVAL TUBE BANKS USING

Temperature contours for Oval tubes at Reynolds number =850 for 3 vortex generator configuration at defined level for 15º angle of attack. Temperature contours for oval tubes at Reynolds number =850 for 7-vortex generator configuration at defined level for 15º angle of attack.

LIST OF GRAPHS

LIST OF TABLES

ABSTRACT

CHAPTER ONE

INTRODUCTION

GENERAL
BACKGROUND OF HEAT TRANSFER ENHANCEMENT

Active heat transfer enhancement techniques
Passive heat transfer enhancement techniques

VORTEX GENERATORS
OBJECTIVES

Recently, controlling the fluid flow in convective heat transfer became a priority topic to improve heat transfer. Electrostatic field is used for dielectric fluids to cause proper bulk mixing of the fluid near the heat transfer surface.

Figure 1 – Example of (a) twisted tapes (b) twisted coils [1]

CHAPTER TWO

LITERATURE REVIEW

Simulation Methods

They indicated that the disturbance of the boundary layer caused the best heat transfer enhancement in the region where the flows are directed towards the wall, but the vortex core is the region of relatively lower mixing. The scaled-up model experiments were conducted to evaluate the heat transfer coefficient and pressure drop, and the prototype experiments were also conducted to investigate the validity of the scaled-up experiments. The J-factors and f-factors for the heat exchanger with DWVG were 87% and 53%, respectively, better than for the finned heat exchangers. The airside heat transfer and friction characteristics of 5 types of fins were analyzed and reported by Tang et al.

40] focuses on the influence of different parameters of VGs on the heat transfer and fluid flow characteristics of three rows of banks of oval tubes. The characteristics of the average Nu number and the skin friction coefficient are studied numerically using computational fluid dynamics (CFD). The results showed an increase in heat transfer and skin friction coefficient with increasing Re number and decreasing relative distance of positions of LVGs. They found that the heat transfer coefficient of the 2D VG could be higher than that of the smooth tip by a factor of 1.8.

Consequently, due to the low inlet velocity and small fin pitch (in 3-D), the flow in the compact heat exchanger channel is assumed to be laminar and uniform.

Figure 10 - Winglet locations and geometry [19]

FLOW SOLVERS

Pressure-Based Solver
Pressure-Velocity Coupling Scheme
Discretization of Momentum Equation (Second-Order Upwind Scheme)

Often this will be one of the additional modeling parameters that limit convergence; in this case SIMPLE and SIMPLEC will give similar convergence rates. Linearizing the discretized equations and solving the resulting system of linear equations to yield updated values of the dependent variables. The pressure-based solver uses a solution algorithm where the governing equations are solved sequentially (i.e. separated from each other).

Solve the pressure correction equation using the newly obtained velocity field and the mass flux. Solve the equations for additional scalars, if any, such as turbulent quantities, energy, species, and radiation intensity using the current values of the solution variables. Pressure-velocity coupling is achieved by deriving an additional condition for pressure by reformulating the continuity equation. The pressure-based solver allows the flow problem to be solved in either a discrete or coupled manner.

When second-order accuracy is desired, quantities at cell levels are calculated using a multidimensional linear reconstruction approach. In this approach, higher-order accuracy at cell levels is achieved by a Taylor series expansion of the cell-centered solution around the cell centroid.

Figure 21 - Flow chart of the Pressure-Based Solution Method Update properties

COMPUTATIONAL DOMAIN AND BOUNDARY CONDITION .1 Circular and Oval Tubes

Aspect Ratio of Oval Tubes

In the case of oval tubes, the cross-sectional area of the tube is the same as that of circular tubes as shown in Figure 23, and the major and minor radii are calculated based on different aspect ratios of the oval tubes. The position of the arm is fixed relative to the center of the circular and oval tube. 32 Figure 26- Orthographic domain view with CFD configuration of winglets (a) Top view. b) Front View 3.4.2 Number of Vortex Generators and Size.

Without vortex generators, we did not consider the base case and 1Vg, 3Vg, 7Vg for the other three different cases. The ratio between the major and minor radii of an oval tube is called the aspect ratio. A numerical investigation was carried out for different aspect ratios of oval tubes for the specified conditions.

Most of the results are considered for AR=1.8 for oval tubes with comparison of circular tubes as shown in Figure 27.

Figure 23 - Schematic of the tube region of a fin-and-tube heat exchanger (a) circular tube (b) oval tube

Mesh Generation

Grid Independency

Pressure drop is a hydraulic loss due to the roughness of the surface over which the liquid flows. Average cross-sectional pressure and pressure drop are defined as: .. where dA is the elementary area A = total area of the selected area. Mean cross-sectional temperature and logarithmic mean temperature are defined as:. where dA is the area of the element.

Where Tw=pipe wall temperature, Tin=air inlet temperature, Tout=air outlet temperature. e) Heat flux (Q). To calculate the surface goodness factor (j/f), we need to determine the friction factor (f) based on the simulated pressure drop results. 38 The Stanton number, St, is a dimensionless number that measures the ratio between the heat transferred in a fluid and the thermal capacity of the fluid.

It appears when considering the geometric similarity of the momentum boundary layer and the thermal boundary layer, where it can be used to express the relationship between the shear force on the wall and the total heat transfer on the wall.

CHAPTER FOUR

RESULTS AND DISCUSSIONS

VALIDATION

40 pressure drop and the numerical values are less than 1%, and the average discrepancy between the predicted airside heat transfer coefficient and the numerical values is less than 20% as seen in Figure 32. The good agreement between the predicted and numerical results indicates that the numerical model is reliable for predicting heat transfer characteristics and flow structure in compact heat exchangers. To verify the reliability of the numerical method used, the numerical simulation is carried out for a fin-and-tube heat exchanger with the model presented in the above figure 33. The peak speed is 16.4% lower in the case of a baseline model (a ) , the top speed is 3.6%.

TWO DIMENSIONAL PRESENTATION OF THE PRESENT WORK

Effect of Tube Banks

A high-velocity region is formed along the side of the tube, as shown in Figure 35(a). Comparing Figure 35(a) with Figure 35(b), we see that the velocity distribution and vortex structure for the inline-7RWP case are different from those for the base case. The winglet pair develops longitudinal vortices that slow down the boundary. layer separation of each tube. A comparison of Figure 36(a) with Figure 36(b) shows that the temperature distribution near the inlet region is almost identical for the baseline inline-7RWP cases.

At the same time, the wake zone behind each tube is reduced by adding winglets (marked with rings).Thus; the temperature behind the RWPs for the inline-3RWP case is significantly lower than in the corresponding region for the baseline case. 44 both baseline and enhanced configurations, the variations in the temperature distribution are almost identical up to the location of the first pipe. In the case of baseline and inline-3RWP, the recirculation area is seen at the back of the pipe for both circular and oval pipes.

In the case of wing sticking, more fluid tends to pass through the tube surface, thus creating a partial vacuum at the rear end of the wing and causing a large recirculation zone which disrupts the thermal and velocity boundary layers and intensifies mixing between the heat. and cold liquids.

Figure 35- Velocity distributions in the channel for the a) baseline case and b) the inline- inline-7RWP (circular tube banks) case c) the inline-inline-7RWP (oval tube banks) case for Re =1775 at 45º angle of attack

Effect of Number of Vortex Generators

Heat Transfer Coefficient, Pressure Drop and Nusselt Number for Different Re Numbers
Effect of Angle of Attack
Effects of Vortex generators Configuration
Effect of Reynolds Number
Number of Vortex Generators
Effect of Tube shape (Circular/Oval)
Effect of Aspect Ratio of Oval Tubes

We note that an increase in the Reynolds number increases the heat transfer coefficient for all the cases listed. The oval tube with 7 vortex generators at higher Reynolds number shows a higher heat transfer capacity than the oval tubes at low Reynolds number. We can see that an increase in the Reynolds number increases the heat transfer coefficient for all the cases listed.

It is found that increasing the Reynolds number increases the pressure drop and heat transfer coefficient for all mentioned cases. It turns out that an increasing number of vortex generators increases the heat transfer coefficient for all mentioned cases. Round tube with vortex generators exhibits higher heat transfer capacity than that of oval tubes.

It is seen that increasing vg number increases the heat transfer coefficient for all the cases indicated. A small heat transfer area due to the shape of the tube is observed in Figure 68. But the area of the heat transfer area is smaller in oval tubes than circular ones.

Figure 39- Performance parameters (a) heat transfer coefficient and the (b) pressure drop for a range of Re numbers for both Circular and Oval tubes

CHAPTER FIVE

CONCLUSION

The circular tube with a 25 degree angle of attack offers a 27% increase in heat transfer over oval tubes with a 15 degree angle of attack. However, for a given tube shape (circular or oval) the increase in heat transfer is approximately 12.5% due to changes in the angle of attack from 15 to 25, oval tubes show only 40% pressure drop while it is 62% for circular tubes. In the reduced AR of oval tubes, both heat transfer and pressure drop increase due to the large frontal area assumed by the small tube diameter.

The aspect ratio of 1.24 gives a remarkable heat transfer improvement of 26.5% with only 69% more pump power than AR=2.34. In the case of 7VG, the friction factor decreases by about 33% for the increase in Reynolds number from 500 to 850. For the Reynolds number ranging from 500 to 850, in the case of 1-RWP (Rectangular Winglet Pair), the increase of y/f ratio is 27% over 7RWP cases.

From the above results, it could be concluded that increasing number of winglets, winglets with higher angle of attack, oval tubes with lower aspect ratio and higher Reynolds number are preferred for better heat transfer with minimum pressure loss.

CHAPTER SIX

FUTURE RECOMMENDATION

C., "Local heat transfer and pressure drop for finned-tube heat exchangers with oval tubes and vortex generators," ASME J. S., "Heat Transfer enhancement for Finned-Tube Heat Exchangers with winglets," ASME Journal Of Heat Transfer, 127, pp. 29] Hiravennavar SR, Tulapurkara EG, Biswas G., “A note on the flow and heat transfer enhancement in a channel with built-in wing pair”, International Journal of Heat and Fluid Flow.

Q., “A three-dimensional numerical study of flow and heat transfer enhancement using vortex generators in fin-and-tube heat exchangers”. Thole, “Increasing heat transfer along the tube wall of a louvered heat exchanger using practical delta fins,” International Journal of Heat and Mass Transfer 51, p. Kasana, “Calculation of Heat Transfer Enhancement in a Plate Fin Heat Exchanger with Triangular Inserts and Triangular Wing Vortex Generator”, Int.

37] Kannan K.T., Kumar B.S., “Heat transfer and fluid flow analysis in a plate-fin and tube heat exchangers with different shaped vortex generators”, Int.

APPENDIX

Bar charts for overall results

Pressure Distribution

NUMERICAL STUDY OF FORCED CONVECTION HEAT TRANSFER OVER CIRCULAR AND OVAL TUBE BANKS USING RECTANGULAR WINGLET VORTEX

GENERATORS