Experimental investigation of turbulent heat transfer and flow characteristics of SiO₂/water nanofluid within helically corrugated tubes

Abstract

Experiments were conducted to investigate the effect of nanofluid on turbulent heat transfer and pressure drop inside concentric tubes. Water and SiO₂ with mean diameter of 30 nm were chosen as base fluid and nano-particles, respectively. Experiments were performed for plain tube and five roughened tube with various heights and pitches of corrugations. Results show that adding the nano-particles in tube with high height and small pitch of corrugations augments the heat transfer significantly with negligible pressure drop penalty. It is discussed on relative Nusselt number and thermal performance of heat exchanger.

Introduction

Heat transfer enhancing techniques are used in heat exchanger to reduce and promote the size and performance, respectively. Generally, there are two group techniques for augmenting the heat transfer: passive and active techniques. One of the best passive techniques is based on introducing surface roughness that disturbs the boundary layer and promoting the turbulence near surfaces. Several roughened tube such as finned tubes, helical tubes, fluted tubes and elliptical axis tubes were investigated by many researchers [1], [2], [3], [4], [5], [6], [7], [8]. Among them, the helically corrugated geometries have been most considered. Helically corrugation creates the chaotic flow mixing and reducing the thickness of thermal boundary layer that increases the heat transfer.

Vicente et al. [9], [10] performed several experiments for laminar, transient and turbulent flows in the HCT with different corrugation heights and pitches. They measured the isothermal friction factor and Nusselt number. Results show that heat transfer does not sensitively change in laminar flow while it intensifies in the turbulent flow. They represented this phenomenon that helical corrugation as roughened surfaces increase flow turbulence and mixes in the near‐wall flow, which is effective in turbulent flow but not in laminar flow. Naphon et al. [11] conducted experimental study on the heat transfer and friction factor in horizontal double pipes using helical ribbed tube. They investigated the effect of relative height and pitch of corrugations on the heat transfer and pressure drop. They used helical ribbed tube with diameter lesser than 10 mm against of other researchers. In agreement with others, they found that the height of corrugation has more significant effect in respect to corrugation pitch on heat transfer and pressure drop. Laohalertdecha and Wongwises [12] studied the effect of corrugation pitch on the condensation heat transfer coefficient and pressure drop of R-134a inside a horizontal HCT. Result indicates that the corrugation pitch has significant effect on heat transfer which increases by increasing the pitch to diameter ratio. Petkhool et al. [13] investigated the turbulent heat transfer enhancement in a concentric tube heat exchanger with helically corrugated tube as inner tube. They studied the effect of three different piths to diameter ratio and three different heights to diameter ratio on heat transfer and isothermal friction augmentation. They proposed a correlation base on their results for Reynolds number ranging from 5500 to 60,000. Another passive method as alternative heat transfer enhancing method is related to adding the nano-particles to based fluid. It increases the heat transfer through with enhancing the thermal conductivity of nanofluid. This method is vastly studied experimentally and numerically by many researchers in various geometries, especially in the heat exchanger because of its extensive application in engineering industries.

Recently, many researchers focused on using the two heat transfer enhancing techniques simultaneously, for example using coil insert and twist tape with nano-particles in circular tube [14], [15]. Wongcharee and Eiamsa-ard [16] investigated experimentally the enhancement of heat transfer using CuO/water nanofluid and twisted tape with alternate axis. They performed experiments for laminar regime in Reynolds number ranging from 830 to 1990 and concentration of nano-particle ranging from 0.3% to 0.7% by volume. Results indicate that by employing the 0.7% volume fraction of nano-particle and twist tape simultaneously promotes the thermal performance by factor of 5.53 for Reynolds number of 1990. Suresh et al. [17] conducted an experimental study on heat transfer and isothermal friction characteristics of CuO/water nanofluid with low concentration in dimpled tube. They performed a comparison of heat transfer between plain tube/water and dimpled tube/nanofluid indicating enhancing up to 27% for the second case. Wongcharee and Eiamsa-ard [18] investigated the effect of CuO nano-particles on heat transfer from corrugated tube equipped with twist tape.

Present experiment deals with the enhancement of the heat transfer thorough with helically corrugation and adding the nano-particles to the base fluid. The effect of different volume concentrations of nano-particles on heat transfer from HCT with various heights and pitches of corrugation are discussed in following section.