(142cg) Turbulent HEAT Transfer in A Microfin TUBE with Twisted Tape Insert

Smith Eiamsa-ard and Vichan Kongkaitpaiboon

Mahanakorn University of Technology, Bangkok, Thailand

Petpices Eiamsa-ard and Monsak Pimsarn

King Mongkut’s Institute of Technology Ladkrabang, Ladkrabang, Thailand

Biography

Smith Eiamsa-ard is an Associate Professor of Mechanical Engineering at Mahanakorn University of Technology, He obtained his D.Eng. from King Mongkut’s Institute of Technology Ladkrabang

Vichan Kongkaitpaiboon is an Assistant Professor of Mechanical Engineering at Mahanakorn University of Technology, He obtained his D.Eng. from Mahanakorn University of Technology

Petpices Eiamsa-ard is D.Eng. candidate at King Mongkut’s Institute of Technology Ladkrabang

Monsak Pimsarn is an Assistant Professor of Mechanical Engineering at King Mongkut’s Institute of Technology Ladkrabang, He obtained his D.Eng. from University of Connecticut

Abstract

The purpose of the present study is to investigate experimentally the heat transfer, turbulent flow friction and thermal performance factor characteristics in a microfin tube with a twisted tape insert.  In the experiment, heat transfer augmentation is expected from both the turbulence of flow near the tube wall produced by fin surface and the swirling flow generated by the twisted tape.  Effect of the twisted tape at different twist ratios of y/W = 3, 4 and 5 on the heat transfer enhancement characteristics is also reported. In the experimental set-up, cold water was passed through the uniform heat flux tube in a range of Reynolds number between 5000 and 17000.  The experimental results obtained are compared with those from plain tubes. The results show that the thermal performance from using the microfin tube fitted with twisted tape is considerably higher than that from the microfin tube alone. The Nusselt numbers and friction factors are found to be, respectively, 3.0, 4.0 and 5.0 times over the microfin tube alone for the microfin tube combined with twisted tape. In addition, the results demonstrate that as the twist ratio (y/W) decreases, the twisted tape will give better heat transfer enhancement.  The microfin tube combined with twisted tape provides higher thermal performance factor than those the microfin tube alone around 172%, 142% and 124%, respectively, for y/W = 3, 4 and 5.  The results were correlated in the form of Nusselt number as a function of Reynolds number, Prandtl number and twist ratio.

Keywords:heat transfer, twisted tape, microfin tube, turbulent flow, swirl flow, heat exchanger

Introduction

The technology of enhanced heat transfer has received serious attention over past decades; heat transfer augmentation techniques find application mainly in the design of more compact heat exchanger at various industries, especially refrigeration, automotive and chemical process industries.  The enhancement of heat transfer has become an important factor in achieving these goals and has captured the interest of many researchers. Active and passive methods have been used to improve heat transfer in chemical reactors, heat exchanger and in flow systems for several decades. The principle of the passive technique involves in either surface treatment, such as coated surface, rough surface and extended surface or flow manipulation such as swirl flow and putting additive into the flow. Active techniques, which require an extra external power source, include mechanical aids, surface vibration, fluid vibration, electrostatic fields, injection or suction of fluid and jet impingement.  One of the most favorable passive techniques is twisted tape inserts because they are inexpensive and can be easily employed to the existing system. The previous studied found that the twisted tape insert can help to generate the swirling flow and stronger turbulence in tubes. This causes a thinner boundary layer and longer resident time of the flow leading to an increase in the heat transfer coefficient. However, an increase in pressure drop is the penalty of the twisted tape technique.

Microfin tube with twisted tape

In the experiments, a microfin tube was made of copper with mean diameter of 8.88 mm and length of 600 mm. The tube was wound with electrical SWG Nichrome heating wire. The terminals of the Nicrome wire were connected to the varaic transformer.  The electrical output power was controlled via a variac transformer. Fifteen K-type thermocouples were tapped along the tube wall for monitoring the local temperatures of the surface tube wall.  The heating tube and thermocouple were covered with insulation to minimize heat loss to surrounding. The heat transfer and pressure drop experiments were conducted individually. The heat transfer experiment was conducted under a uniform heat flux.  During the heat transfer test, the test tube was heated by continually winding flexible electrical wire. On the other hand, the pressure drop (friction) test was performed under an isothermal condition without turning on the heater.  In the experiments, the twisted tapes were installed into the microfin tube at three different twist ratios of y/W = 3, 4 and 5.

Experimental results

Effect of microfin tube

Effect of using the microfin tube on heat transfer characteristics is presented.  It is found that the microfin tube gives higher heat transfer rate than the plain tube. The maximum Nusselt number increases at about 38% when compared with those from the plain tube.  This can be attributed to better mixing of fluids between the wall using fin-surfaced wall causing the turbulence flow and pressure gradient in the radial direction.  The boundary layer along the tube wall would be redeveloped along the fin wall resulting in more heat flow through the fluid.  Furthermore, the turbulence enhances the turbulence fluctuations, which leads to even better convection heat transfer. Thus, the higher the Reynolds number is, the greater the Nusselt number.  The relationship between the friction factor and Reynolds number for using the microfin tube is presented.  It is worth noting that friction factor of the microfin tube is considerably higher than that of the plain tube. This is because of higher surface area and the dissipation of dynamic pressure of the fluid at high viscosity loss near the tube wall. The friction factor is found to reduce gradually for increasing Reynolds number.  In addition, it is found that the thermal performance factor of the microfin tube decreases with increasing Reynolds number.  The microfin tube alone yields the maximum thermal performance factor at 1.12.  This may be explained by the fact that good contacts of the core fluid with the tube wall from turbulence between the fin elements.

Effect of combined microfin tube and twisted tape insert

It can be seen that use of the microfin tube with the twisted tape yields a higher heat transfer rate than that of the microfin tube alone.  The increase in Nusselt number can be explained by two mechanisms: firstly, the surface of the microfin tube may be act as extended heat transfer surfaces and secondly, it is due to strong turbulence/swirl flow created by the twisted tape.  In general, the average heat transfer rate for microfin tube with the twisted tape is found to be 63 to 98% better than that for the microfin tube alone. The corresponding increase in the mean Nusselt number of microfin tube with the twisted tape is about 124 to 172% over the plain tube.

It is apparent that the use of the microfin tube along with the twisted tape also provides higher friction factor than that of the microfin tube alone due to larger contact surface areas and flow disturbance at the core.  Besides, the presence of the microfin tube together with twisted tape reduces flow areas, resulting in a high speed rotating flow.  This leads to the substantial pressure loss action of the fluid higher than one without the twisted tape.  For the microfin tube with the twisted tape, the increase in friction factor is found to be around 171 to 262% above one without the tape.  It is reveal that the mean friction factors of using microfin tube fitted with twisted tape are found to be about 3.6, 3.1 and 2.7 times for the y/W = 3.0, 4.0 and 5.0, respectively, over the microfin tube alone.

The influence of microfin tube with a twisted tape insert on thermal performance factor is presented.  It can be observed that use of the microfin tube fitted with the twisted tape leads to higher thermal performance than those of the microfin tube alone.  Thermal performance varied between 1.17 and 1.52, 1.10 and 1.40, and 1.05 and 1.35 for the microfin tube with twisted tape at y/W = 3, 4 and 5, respectively. The thermal performance tends to reduce with the increase of Reynolds number. The microfin tube with twisted tape shows a faster decrease in the thermal performance than the others for increasing Reynolds number. Close examination reveals that the thermal performances for all enhancement devices are higher than unity.  This suggests that the thermal performance with/without the twisted tape is not feasible in terms of energy saving, especially at higher Reynolds numbers.