3-D Numerical study on the correlation between variable inclined fin angles and thermal behavior in plate fin-tube heat exchanger

https://doi.org/10.1016/j.applthermaleng.2007.01.011Get rights and content

Abstract

In this study, the heat transfer enhancement and pressure drop values of seven different fin angles with plain fin-tube heat exchangers were investigated. The numerical simulation of the fin-tube heat exchanger was performed by using a three dimensional (3-D) numerical computation technique. Therefore, a CFD computer code, the FLUENT was used to solve the equation for the heat transfer and pressure drop analyses in the fin-tube heat exchanger. The model drawing was created and meshed by using GAMBIT software. The heat transfer and pressure drop values of the vertical fin angle (θ = 0°) were provided to compare with variable inclined fin angles (θ = 5°, 10°, 15°, 20°, 25°, 30°). The heat transfer values were normalized to compare all cases. For inclined fin angle θ = 30°, which is the optimum angle, the maximum heat transfer enhancement per segment was obtained 1.42 W (the normalized value 105.24%), the maximum loss power associated with pressure drop per segment was only 0.54 mW.

Introduction

Heat exchangers are widely used devices in the industry because of supply heat transfer between two fluids that are at different temperatures and separated by a solid wall. The plate fin-tube heat exchangers have been used in the thermal engineering applications, such as power station, chemical plants, food industries, heating–cooling systems, aircrafts, automotive sectors, etc. There are various fin types that used plate, louver, convex louver, wavy and tube geometries that used circular, elliptical, etc. The plate fin shapes are still the most popular fin pattern in the fin-tube heat exchanger applications because of its durability, simplicity, versatility and rigidity. In this case, many researchers have studied to effectively improve of the fin-tube heat exchangers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. They have made experimentally and numerically investigation on heat transfer performance and pressure drop characteristics of the plate fin-tube heat exchangers.

Experimental study of the plain fin-tube heat exchangers is very useful but very expensive due to because of the high cost of the tools. Nevertheless, some studies have been done experimentally to determine heat transfer and friction characteristics of the plate fin-tube heat exchangers. Wang and Chi have intended to provide new experimental data for the plain fin-tube heat exchangers owing to the present authors need further experimental information [7]. Tao et al. have studied experimentally and numerically 3-D model of air side heat transfer and pressure drop. They have compared the performance of slotted fin and plain plate fin surface [13]. Kim et al. have investigated a new experimental-numerical method for the evaluation of thermal contact resistance in the fin-tube heat exchangers [14], [15].

On the other hand, the general theory of fluid motion is sometimes difficult to enable the user to apply arbitrary geometric configurations. It can be possible to achieve only numerical techniques to arbitrary geometries. The second advantage of numerical method is to apply without the experimental study or reducing the extent and number of experiments required to describe of flow. Therefore, a suitable numerical method and/or computational fluid dynamics (CFD) code is frequently used to solve the governing equations of fluid flow. The CFD code provides to predict some information about the flow speed, pressures, residence times, flow patterns, heat transfer, etc. Thus, numerical methods have been performed by several researchers for the analysis of the fin-tube heat exchangers included in several geometries and different boundary conditions. Romero-Mendez et al. have investigated numerically the effect of fin spacing on convection in a plate fin-tube heat exchanger [2]. Erek et al., have analyzed a plate fin type heat exchanger with one row tube configuration for different geometrical parameters by using a numerical computation technique. They have also investigated numerically the effects of the distance between two fins, tube center location, fin height, tube thickness and tube ellipticity on heat transfer and pressure drop across the heat exchanger [1]. Haught and Engelmann have used a finite element method and reported examples of velocity and temperature fields [16]. Fiebig et al. have reported on the results of finite volume calculations of the flow and conjugate heat transfer in fin-tube geometry. They have calculated the flow variations, pressure drop, Nusselt number distribution and fin efficiency as a function of Reynolds number [17]. Jang et al. have reported a numerical calculated of Nusselt number and pressure drop as a function of the fin-spacing based on Reynolds number [18].

Reviewing the literature, it has been found that the effect of geometrical parameters on heat transfer and pressure drop for the various fin types and tube geometries of the fin-tube heat exchangers have been investigated during the recent decades. However, the effective factors of inclined fin angles on heat transfer and pressure drop across in a heat exchanger have not been analyzed numerically. Having more basic structure, variable inclined fin angle systems are easer to fabricate than the flue gas flow angle systems such as louvered fins. In fact, manufacturing of louvered fin geometries changing flue gas flow angles used for heat transfer enhancement in the heat exchangers is more complicated and expensive than the fin angle systems. The cutting force increases due to having more and more louvers on the fin. So this situation requires using more press capacity. In addition, increasing the number of punches enlarges dimensions of the die which will raise cost of manufacture. Moreover, having more operations result in more workers’ pay. On the other hand cutting die method facilitates fabricating of the fin angles. Angle of hole on the plate fin is formed by cutting die methods. There are two cutting die methods to obtain any desired angle hole. These are cam-punching die method in which the direction of elements is at an angle to the direction of forces supplied by a press and shaving die method having angled cutting edges. More detail can be found in Ref. [19]. These variable fin angle systems using the same number of fins as straight flue gas systems supply more heat transfer enhancement. In case of reducing fin spacing for straight flue gas direction more fin numbers will be required to provide the same heat transfer enhancement. In this respect, variable inclined fin angles used in the plain plate-tube heat exchangers were considered to investigate. As differing from other work, the main objective of this study is to analyze for different inclined fin angles by using a three dimensional (3-D) numerical computation technique, the help of the FLUENT, a CFD computer code [20]. The heat transfer performance comparison of the vertical fin angle and variable inclined fin angles having plain plate-tube heat exchangers have been tabulated and discussed.

Section snippets

Mathematical modeling

The governing equations for continuity, momentum and energy in the computational procedure can be written as follows.

  • Continuity equation:xi(ρui)=0.

  • Momentum equation:xi(ρuiuj)=xiμujxi-pxj.

  • Energy equation:xi(ρuiT)=xikcpujxi.

These governing equations in the Cartesian coordinates are the elliptic form. In the equations, ρ is density, u velocity, μ dynamic viscosity, p pressure, k thermal conductivity, T temperature and cp specific heat.

Calculation procedure

The numerical simulation of the fin-tube heat

Numerical results and discussion

The FLUENT was run for each model to obtain numerical results. The heat transfer from the flue gas in the gap between two fins to the water inside the tube and total pressure drop values of flue gas across the heat exchanger were calculated numerically for each model. Then, obtained results were presented in Table 1. Here the heat transfer rate from the flue gas on the distance between two fins of the tube surface into the water inside the tube (Q˙1) is not important value due to its very small

Summary and conclusions

Different inclined fin angles on the heat transfer between flue gas and water, and pressure drop of flue gas passing through two fins were evaluated numerically. Then, the calculated results have shown that the applied models are achieved successfully and the conclusions can be summarized as follows:

  • The FLUENT was applied successfully for the heat transfer and pressure drop analyses in the fin-tube heat exchanger.

  • The results indicated the effective enhancement of the heat transfer performance.

Acknowledgements

The authors gratefully acknowledge Gazi University and Erciyes University for their support in this study.

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