Pressure drop and heat transfer comparison for both microfin tube and twisted-tape inserts in laminar flow

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Abstract

Experiments were carried out to compare pressure drop and heat transfer coefficients for a plain, microfin, and twisted-tape insert-tubes. The twisted-tape experiments include three different twist ratios each with two different widths. The data were taken at Reynolds numbers well in the laminar region. The heat transfer data were obtained in a single shell-and-tube heat exchanger where steam is used as a heat source to obtain a uniform wall temperature and the working fluid in the tube is oil. The twist ratio and the width of the tape seem to have a large effect on the performance of the twisted-tape insert. The results demonstrate that as the twist ratio decreases, the twisted-tape will give better heat transfer enhancement. The loose-fit (W=10.8 mm) is recommended to be used in the design of heat exchanger where low twist ratios (Y=5.4, and Y=3.6) and high pressure drop situations are expected since it is easier to install and remove for cleaning purposes. Other than these situations, the tight-fit tape gives a better performance over the loose-fit tape. For the microfin tube tested in this paper, the data shows a small increase in both heat transfer and pressure drop. This type of microfin tube is not recommended to be used in laminar flow conditions.

Introduction

Many studies were conducted previously to analyse heat transfer and pressure drop of both twisted-tape-inserts and finned tubes. Most of the early work was concerned with the effect of twisted-tape on turbulent flow conditions 1, 2, 3. Du Plessis [4] conducted a numerical and experimental heat transfer study, respectively, for laminar isothermal boundary layer condition with twisted-tape-insert. Hong and Bergles [5] correlated heat transfer and pressure drop data for twisted-tape-insert for uniform wall temperature conditions using distilled water and ethylene glycol as the working fluids. Manglik and Bergles 6, 7 also reported experimental data for twisted-tape. Their pressure drop data indicated that the friction factor depends primarily on the Reynolds number and swirl parameter (Sw=Re2/Y). The increase in the swirl flow gives a resultant increase in the friction factor. In addition, Manglik and Bergles [7] correlated the heat transfer effect of twisted tape for free and forced convection. Bandypadhyay et al. [8] carried out heat transfer experiments on twisted-tape-insert in mixed convection. Chakroun and Al-Fahed [9] studied the effect of twisted-tape width on both heat transfer and pressure drop for fully developed laminar flow.

Microfin tubes are routinely used to provide enhancement for tube-side refrigerant condensation and evaporation. However, the potential for using microfin tubes for single-phase flow has been gaining momentum. This tube, illustrated in Fig. 1(a), has triangular cross-section fins 0.2–0.35 mm high at helix angles between 8 and 30 (measured from the tube center line). Brognaux et al. [10] reported heat transfer and friction characteristics for single-phase flow in single-grooved and cross-grooved microfin tubes. Their results showed that the microfin tubes provided heat transfer enhancement as high as 1.8 times that of the plain tubes. Most of data reported by Brognaux et al. were taken for turbulent flow. Data for laminar flow will be reported here for the complete understanding of the microfin tubes.

The present work investigates both microfin and twisted-tape enhancement techniques from both heat transfer and pressure drop point of views. The work also investigates the effect of the twisted-tape width on the enhancement of heat transfer by comparing the results of two different twisted-tape widths. The two twisted-tape widths considered in this study give width ratios (twisted-tape width to the inside diameter ratio) of 0.95 and 0.77. The 0.95 width ratio is considered to be the tight-fit, where as the 0.77 width ratio is considered to be the loose-fit. The work will investigate the conditions where the loose-fit twisted-tape can be used since it is easier for installing and cleaning over the tight-fit one.

A comparison is carried out on both pressure drop, and heat transfer for plain tube, microfin tube, and tube with twisted-tape-inserts. The twisted-tape-insert experiments include three different twist-ratios each with two different tape widths. The experiments are conducted in the laminar flow region and at uniform wall temperature conditions. The objective of the work is to investigate the use of twisted-tape-insert and the microfin tube in the design of shell-and-tube heat exchangers, as well as to analyze their thermal performance in order to reduce the operating cost. The thermal improvement results in an increase in the pumping power caused by the increase of pressure drop.

Section snippets

Experimental procedure

The experimental work is conducted to reach a more general and practical look at the effect of microfin and twisted-tape-insert tubes for both heat transfer and friction factor coefficients. Data are collected for fully developed laminar flow under a uniform wall temperature conditions. The test fluid used in the experiment is a high viscosity oil which gives values of Reynolds number well in the laminar region. Steam is used as the heating medium to achieve a uniform wall condition. The

Uncertainty analysis

The uncertainty in the experimentally determined friction factor and Nusselt number were estimated based on the procedures of Coleman and Steele [11] and ANSI/ASME standard [12]. The uncertainty U in the measured value of either Nusselt number or friction factor is expressed as follows.U=(Eb2+Ep2)1/2,where Eb is the bias error and Ep is the precision limit error in the measured value. The expressions used in the determination of the friction factor and Nusselt number are given in , ,

Plain tube qualification

The plain-tube data serves as a qualification for the facility and the procedure used over the range of Reynolds number anticipated. Pressure drop and heat transfer data were collected and the results were compared to known correlations. Fig. 5 presents a comparison for the friction factor results calculated from the pressure drop data using Eq. (5)with the analytical solution of the laminar flow in plain tube. The data show an excellent agreement with the 64/Re equation. Heat transfer data are

Pressure drop and heat transfer results

The comparison is carried out on plain, microfin, and twisted-tape-insert tube with three twist ratios of 3.6, 5.4, and 7.1; each having two tape-widths of 13.2 and 10.8 mm. Both tight-fit and loose-fit twisted tapes were investigated. The 13.2 mm-width twisted tape is inserted with a tight-fit since the width is close to the inside diameter of the tube. The 10.8 mm-width twisted tape is a loose-fit and the tape is held straight throughout the experiments. The loose-fit was investigated since

Conclusion

An experimental work has been performed to compare both heat transfer and pressure drop results for plain, microfin, and twisted-tape-insert tubes. The twisted-tape-inserts include three twist ratios and two different widths. The effect of width from both heat transfer and pressure drop point of views is also discussed. The data were collected for Reynolds number in the laminar region. Oil is used as the working fluid and steam is used as the heating source to obtain a uniform wall temperature.

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