Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator

Abstract

Friction and compound heat transfer behaviors in a dimpled tube fitted with a twisted tape swirl generator are investigated experimentally using air as working fluid. The effects of the pitch and twist ratio on the average heat transfer coefficient and the pressure loss are determined in a circular tube with the fully developed flow for the Reynolds number in the range of 12,000 to 44,000. The experiments are performed using two dimpled tubes with different pitch ratios of dimpled surfaces (PR = 0.7 and 1.0) and three twisted tapes with three different twist ratios (y/w = 3, 5, and 7). Experiments using plain tube and dimpled tube acting alone are also carried out for comparison. The experimental results reveal that both heat transfer coefficient and friction factor in the dimpled tube fitted with the twisted tape, are higher than those in the dimple tube acting alone and plain tube. It is also found that the heat transfer coefficient and friction factor in the combined devices increase as the pitch ratio (PR) and twist ratio (y/w) decrease. In addition, an empirical correlation based on the experimental results of the present study is sufficiently accurate for prediction the heat transfer (Nu) and friction factor (f) behaviors.

Introduction

A large portion of energy being consumed in industry processes and the energy resources are depleting at an alarming rate. Energy conservation is therefore, become an important issue. In many areas of the industries, using of high-performance heat exchanger is one of the promising energy-saving manners.

The high-performance heat exchangers can be obtained by utilization of heat transfer enhancement techniques. In general, heat transfer enhancement creates one or more combinations of the following conditions that are favorable for the increase in heat transfer rate with an undesirable increase in friction: (1) interruption of boundary layer development and rising degree of turbulence, (2) increase in heat transfer area, and (3) generating of swirling and/or secondary flows [1]. Several enhancement techniques have been introduced, for example, treated surfaces, rough surfaces, swirling flow devices, coiled tubes, and surface tension devices [2]. Numerous works have revealed that dimpled tube and twisted tape are effective devices for enhancing heat transfer rate. Here, some of the related works are described as follows. Rabas et al. [3] reported the influence of roughness shape and spacing on the performance of three-dimensional helically dimpled tubes. Chen et al. [4] performed an experimental work to study the heat transfer coefficient, friction factor and enhancement index characteristics in a dimpled tube at different depth/pitch of dimples. Their results showed that the heat transfer rates were enhanced from 25% to 137% at constant Reynolds number, and 15% to 84% at constant pumping power. Vicente et al. [5] reported the effects of the three-dimensional helically dimpled tubes (dimpled height, h/d = 0.08 to 0.12 and helical pitch, p/d = 0.65 to 1.1) on the heat transfer and isothermal friction in turbulent flow region. Vicente et al. [6] also conducted an experimental work to determine the heat transfer and friction characteristics in dimpled tubes in laminar and transition flow regions using water and ethylene glycol as working fluids. They observed that laminar flow heat transfer through horizontal dimpled tubes is produced in mixed convection, where Nusselt number depends on both the natural convection and the entry region.

Insertion of twisted tape is one of the effective methods to increase heat transfer coefficient with relatively small pressure drop penalty. Twisted tape has been used to create swirling flows that modify the near wall velocity profile due to the various vorticity distributions in the vortex core. The fluid mixing between the tube core and the near wall region is enhanced because of the swirl induce tangential flow velocity component [1]. The twisted tape and other heat transfer enhancement devices were also utilized simultaneously to gain better results compared to that by a single device. This approach is known as compound enhancement [2]. Al-Fahed et al. [7] carried out an experimental work to study the heat transfer coefficients and friction factors in a microfin tube fitted with twisted tape for three different twist/width ratios under laminar flow region. Liao and Xin [8] conducted an experimental study to determine the heat transfer and friction characteristics in tubes with three-dimensional internal extended surfaces combined with a twisted tape using various working fluids (water, ethylene glycol, and ISO VG46 turbine). Zimparov [9] combined the single start spirally corrugated tubes with the twisted tape inserts for their heat transfer enhancement. Pramanik and Saha [10] investigated the heat transfer and friction loss for laminar flow with viscous oil through rectangular and square ducts with internal transverse rib turbulators on two opposite surfaces of the ducts and fitted with twisted tapes. The tapes used were full length, short length, and regularly spaced types. They found that the transverse ribs in combination with full-length twisted tapes performed better than either ribs or twisted tapes acting alone. The pressure drop and compound heat transfer characteristics of a converging–diverging tube with evenly spaced twisted tapes were considered by Mengna et al. [11]. Their results showed that the heat transfer rates were 0.85 to 1.21 times of those in a plain tube and 1.07 to 1.15 times of those in a converging–diverging tube without twisted tape inserts. Promvonge and Eiamsa-ard [12] investigated the heat transfer, friction factor and thermal enhancement characteristics in a tube with combined conical-ring turbulator and twisted tape. Recently, Promvonge [1] reported the influences of wire coils in conjunction with twisted tapes on the heat transfer and friction factor in a uniform heat-flux tube.

The advantages of the dimpled tube, twisted tape and compound heat transfer enhancement, shown in literature review above, motivate us to investigate heat transfer enhancement by using the dimpled tube together with the twisted tape as compound enhancing device. In the present work, the effects of pitch and twist ratios on the heat transfer coefficient and pressure loss characteristics in the fully developed turbulent flow of a dimpled tube with a twisted tape insert are examined. The Reynolds numbers are ranged from 12,000 to 44,000 with hot/cold water as working fluid. The experimental results of the heat transfer enhancement and pressure loss as well as the empirical correlation for Nusselt number and friction factor are presented.