Journal of Thermodynamics

Volume 2015 (2015), Article ID 368960, 18 pages

http://dx.doi.org/10.1155/2015/368960

## The Influence of the Punched Delta Wings on Flow Pattern and Heat Transfer Characteristic in a Fin-and-Oval-Tube Heat Exchanger

Department of Mechanical Engineering Technology, College of Industrial Technology, King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand

Received 7 July 2015; Revised 22 September 2015; Accepted 29 September 2015

Academic Editor: Felix Sharipov

Copyright © 2015 Amnart Boonloi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

3D numerical investigations are performed to study the heat transfer, friction factor, and thermal performance of a fin-and-oval heat exchanger with punched delta wings for a range of 500 ≤ Re ≤ 2500 (based on the hydraulic diameter). The influences of the punched angles, 20°, 30°, and 45°, flow directions, wing tips pointing downstream and upstream, and pitch ratios, 2, 3, 4, 5, and 6, are investigated. The results show that the use of the punched delta wings in the fin-and-oval-tube heat exchanger leads to an enhancement in the heat transfer and friction loss as compared to the plain fin for all cases (/ and higher than 1). The enhancements of the heat transfer and friction factor are around 1.01–1.22 and 1.37–2.65 times higher than the base case, respectively. The punched delta wings create the vortex flows through the test section that helps enhance the strength of the impinging flow on the tube walls. The impingement of the fluid flow is an important key to augment the heat transfer rate and thermal performance in the heat exchanger.

#### 1. Introduction

Many turbulators such as wing and winglet are used to enhance the heat transfer rate and thermal performance in fin-and-tube heat exchangers. The turbulators can change the flow structure of the working fluid in the heating system when comparing with the plain fin. The vortex flow or swirling flow is the flow mechanism when using the turbulators that helps to improve the heat transfer rate and performance. Due to the narrow parallel space between the fins, the pressure loss is an important factor which should be considered when installing with various types of turbulators. The investigations on heat transfer, pressure loss, and thermal performance in the fin-and-oval-tube heat exchanger with using turbulators have been widely reported. The numerical study is a method to investigate the effects of turbulators on heat transfer and thermal performance in the fin-and-oval-tube heat exchanger. The numerical results can help to describe and report flow structure and heat transfer behavior when using the turbulators in the heat exchanger. The understanding of flow and heat transfer profiles is a way to improve thermal performance of the heating equipment. Moreover, the numerical investigation can help reduce cost and time to study in comparison with the experimental method.

For example, Li et al. [1] numerically studied the flow patterns and heat transfer characteristics in a fin-and-tube heat exchanger with longitudinal vortex generators. The punched and mounted rectangular and delta winglets were used as the vortex generators. They concluded that the use of the vortex generators gives the Nusselt number around 20% higher than the plain fin. They also reported that the maximum thermal performance is found at the attack angles of 25° and 45°, respectively, for the rectangular and delta winglets. Gong et al. [2] reported the use of punched rectangular vortex generators in a heat exchanger on thermal performance. They found that the secondary flow and reducing wake region are reasons for enhancing the heat transfer rate and thermal performance. Delač et al. [3] investigated with the numerical method for the augmentation of heat transfer rate in a fin-and-tube heat exchanger with vortex generators at Re = 350–2200. They presented the effects of the impact angles, 5°, 10°, and 20°, and the winglet height. They summarized that the impact angle of 10° performs the optimum ratio between the enhancing heat transfer rates and the increasing friction loss in the heat exchanger. Lotfi et al. [4] numerically studied various type vortex generators, rectangular trapezoidal winglet (RTW), angle rectangular winglet (ARW), curved angle rectangular winglet (CARW), and wheeler wishbone (WW), in a wavy-fin-and-elliptical-tube heat exchanger on flow structures and heat transfer behaviors. The effects of the attack angles, 15°, 30°, 45°, 60°, and 75°, and width/length aspect ratios, 0.5 and 1.0, were reported. They found that the CARW gives the best thermal performance at a small attack angle, while the RTW provides the optimum thermal performance at a large angle. Gholami et al. [5] presented the influences for rectangular winglet vortex generators with the attack angle of 30° on heat transfer and pressure drop in a fin-and-tube heat exchanger for Re = 400–800. They claimed that the 30° wavy rectangular winglet vortex generators can improve the heat transfer rate with a moderate pressure loss penalty. The numerical investigations on the influences of the span angles 30°–60° and transverse locations 2–20 mm for vortex generators in a plate-fin-and-tube heat exchanger at Re = 400–1200 were reported by Jang et al. [6]. He et al. [7] presented the numerical investigations on heat transfer and pressure loss in a fin-and-tube heat exchanger with rectangular winglet vortex generators. They concluded that the longitudinal vortex flow and the impinging flow are important factors to augment the heat transfer rates. They also pointed out that the staggered arrangement of the RWVG can reduce the pressure loss in the heat exchanger by using rectangular winglet vortex generators. Saha et al. [8] studied the influences of rectangular delta winglets with common-flow-up and common-flow-down arrangements in a plate-fin heat exchanger on thermal performance with numerical method. They summarized that the RWP provides higher heat transfer rate than the DWP. Du et al. [9] analyzed the effects of punched longitudinal vortex generators in a wavy-finned-flat-tube heat exchanger. They presented that the optimum thermal performance is found at the attack angle of 25° when using the delta winglet pairs. Huisseune et al. [10] investigated performance evaluations in a louvered fin heat exchanger by using punched delta winglet vortex generators. They found that the phenomena, reducing tube wakes, swirling flow, and vortex flow, which were created by the punched delta winglet vortex generators, can help to improve heat transfer rate and performance. Li et al. [11] found that the use of longitudinal vortex generators in a slit fin-and-tube heat exchanger provides the thermal enhancement factor around 4.2% higher than the base type heat exchanger. Lemouedda et al. [12] studied the effects of the attack angles for delta winglet vortex generators in a plate-fin-and-tube heat exchanger with Re = 200–2000 based on the inlet height. They also compared the numerical results of the winglet locations between inline and staggered arrangements. Du et al. [13] investigated the heat transfer and thermal performance improvement in a wavy-fin-flat-tube heat exchanger with longitudinal vortex generators. They presented that the values of the Nusselt number and friction factor increase around 21–60% and 13–83%, respectively, for Re = 1500–4500. Chu et al. [14] numerically studied flow topologies, heat transfer profiles, and performance improvement for a fin-and-oval-tube heat exchanger with longitudinal vortex generators at Re = 500–2500. They reported that the heat transfer and pressure loss are around 13.6–32.9% and 29.2–40.6% higher than the base case, respectively. They also stated that the best thermal performance is found at the attack angle of 30°, with downstream arrangement with two tube rows. Joardar and Jacobi [15] carried out study on the heat transfer augmentation by using winglet vortex generators in a plain-fin-and-tube heat exchanger for Re = 220–960. They indicated that the augmenting heat transfer rate is around 16.5–44% and 19.9–68.8% for single row and three rows of the winglets, respectively. Li et al. [16] investigated thermal performance in a fin-and-tube heat exchanger with radiantly arranged winglets around the tubes. They claimed that the use of the winglet leads to a reduction in the wake regime behind the tube. Tian et al. [17] analyzed punched delta winglets for both inline and staggered arrangements in a wavy-fin-and-tube heat exchanger with numerical method. They concluded that factor is around 13.1% and 15.4% for the staggered and inline arrangements, respectively. Lawson and Thole [18] presented heat transfer augmentations in a louvered-fin-and-tube heat exchanger with delta winglets. They found the enhancements on the heat transfer rate and pressure loss were around 47% and 19%, respectively.

From the foregoing review, we found that the enhancements of heat transfer and thermal performance in fin-and-tube heat exchangers with winglet type vortex generators has been widely reported. The winglet vortex generators can help generate the vortex flow or swirling flow through the test section which leads to increasing the heat transfer rates. However, the use of the winglet vortex generators not only increases the heat transfer rate but also increases the pressure loss. In current investigation, the punched delta wings are used in a fin-and-oval-tube heat exchanger to enhance the heat transfer rate and performance. The use of the punched delta wings is expected to reduce the pressure loss in the heating system in comparison with the winglet vortex generators. The punched delta wing angles, 20°, 30°, and 45°, flow directions, wing tips pointing downstream and upstream, and pitch ratios, 2, 3, 4, 5, 6, are investigated numerically for Re = 500–2500 based on the hydraulic diameter of the heat exchanger.

#### 2. Computational Domain

Figures 1(a) and 1(b) present the fin-and-tube heat exchanger with the punched delta wings for single fin plate and three fin plates, respectively, while the details of the punched delta wings are displayed in Figure 1(c). The delta wings are punched from the fin surface with different punched angles: 20°, 30°, and 45°. is the computational domain length equal to 64.4 mm. The influences of or PR, 2, 3, 4, 5, and 6, are presented for Reynolds number, Re = 500–2500, based on the hydraulic diameter () of the fin-and-oval-tube heat exchanger. The wing tips are set on pointing both downstream and upstream in this investigation. The smooth entrance regime of the computation domain is around 10 to maintain the inlet velocity uniformity and the domain is extended by 30 at the exit regime to ensure a recirculation-free flow there. The parameters of the computational domain are displayed in Table 1.