Heat and fluid flow characteristics of roughened solar air heater ducts – A review

doi:10.1016/j.renene.2012.04.001

Renewable Energy

Volume 47, November 2012, Pages 77-94

https://doi.org/10.1016/j.renene.2012.04.001 Get rights and content

Abstract

Artificial roughness in the form of repeated ribs is one of the effective way of improving the performance of a solar air heater ducts. Various studies have been carried out to determine the effect of different artificial roughness geometries on heat transfer and friction characteristics in solar air heater ducts. The objective of this paper is to review various studies, in which different artificial roughness elements are used to enhance the heat transfer coefficient with little penalty of friction factor. On the basis of correlations developed by various investigators for heat transfer coefficient and friction factor, an attempt has been made to compare the thermohydraulic performance of roughened solar air heater ducts. It has been found that lot of experimental and analytical studies reported in the literature.

Highlights

► Review and presented various roughness geometries of solar air heater duct. ► Review heat transfer and fluid flow correlations of roughened solar air heater duct. ► Review and discussed PIV, Thermogrphy and CFD methods of solar air heater ducts. ► The study shows that the multi v-ribs are highest Thermohydraulic performance.

Introduction

Energy is a basic ingredient needed to sustain life and development. Energy is needed in various forms to fulfill our daily requirements. Solar energy is available freely and a clean source of energy [1]. The simplest and the most efficient way to utilize solar energy is to convert it into thermal energy for heating applications by using solar collectors [2]. Solar air heaters, because of their inherent simplicity are cheap and most widely used for many applications at low and moderate temperatures.

Artificially roughened absorber plate is considered to be a good methodology to breaking the laminar sub-layer in order to reduce thermal resistance and to increase heat transfer coefficient. Regarding artificial roughness, many experimental investigations have been reported in literature by various authors. In this paper, an attempt has been made to categorize and review the reported roughness geometries used for creating artificial roughness. Correlations for heat transfer coefficient and friction factor developed by various investigators for solar air heater ducts having artificial roughness of different geometries were reviewed and presented in the paper.

Section snippets

Performance of flat plate solar collector

Thermal performance of flat plate solar collector was first investigated by Hottel and Woertz reported by Duffie and Beckman [2].

Bliss [3] introducing ‘collector heat removal factor’, F_R, defined as the ratio of actual useful energy gain to the useful energy gain if the whole collector absorbing surface were at the fluid inlet temperature (Ti). $q_{u} = \frac{Q_{u}}{A_{c}} = F_{R} [I (τ α) - U_{L} (T_{i} - T_{a})]$ where, $F_{R} = \frac{m C_{p} (T_{o} - T_{i})}{A_{c} [I (τ α) - U_{L} (T_{i} - T_{a})]}$

Also $η_{th} = F_{R} [(τα) - U_{L} (\frac{T_{i} - T_{a}}{I})]$

Eq. (3) is known as Hottel–Whillier–Bliss equation. Eq. (3)

Artificial roughness

In order to attain higher heat transfer coefficient it is desirable that the flow at the heat transferring surface is made turbulent. However, energy for creating such turbulence has to come from the fan or blower and the excessive turbulence leads to excessive power requirement to make the air flow through the duct. It is therefore desirable that the turbulence must be created only in the region very close to the heat transferring surface i.e. in the laminar sub-layer only where the heat

Fluid flow and heat transfer characteristics of solar air heater duct with artificial roughness

Efforts for improving the heat transfer rate have been directed towards artificially destroying or disturbing the viscous sub-layer by providing the artificial roughness on heated surface. Many experimental investigations have been carried out to study the flow field and characteristics of heat transfer and friction factor of roughened tubes, annuli and ducts [4], [5], [6], [7], [8] and [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26],

Effect of different roughness parameters on heat transfer coefficient and friction factor

The effect of different roughness parameters, relative roughness pitch (P/e), relative roughness height (e/D), angle of attack (α) on heat and fluid flow characteristics as investigated by various investigators is given below.

Different type roughness geometries in solar air heater ducts

Besides the above-mentioned parameters, shapes of various roughness elements also influence the heat transfer rate and friction factor. Various shapes of artificial roughness geometries are discussed.

PIV and thermography methods

In view of the fact that the flow and temperature distribution resulting from the use of artificial roughness is highly complex, several investigators have attempted studies on the local velocity and temperature distributions in rectangular ducts using artificial roughness for enhancement of performance. These studies have resulted into very useful conclusions regarding the optimization of geometrical parameters of the roughness elements. Some investigator studied the artificial roughness duct

Computational analysis (CFD)

Presently, CFD analysis is considered to be relative tools to analysis the fluid flow and heat transfer characteristics in various systems. Investigators, carried out CFD based analysis in artificially roughened duct which are presented as below:

Chaube et al. [38] carried out a CFD analysis using Fluent 6.1 software to investigate flow and heat transfer characteristics of two-dimensional rib roughened rectangular ducts with one wall subjected to uniform heat flux of 1100 W/m². They used SST K-ω

Enhancement of collector thermal performance

The use of artificial roughness on the underside of absorber plate of the solar air heater leads to considerable enhancement in the heat transfer. This results in similar enhancement in the thermal efficiency of the solar collector [42]. Fig. 33 shows that there is substantial improvement in the thermal efficiency of solar air heater and that this enhancement is a strong function of roughness parameters.

Saini [42] investigated the thermal performance of solar air heater having artificially

Conclusions

In the present paper an attempt has been made to review heat transfer and friction characteristics of artificially roughened duct of solar air heaters. The concept of performance enhancement of roughened ducts has also been discussed. Correlations for heat transfer coefficient and friction factor by various investigators developed for solar air heater ducts having artificial roughness of different geometries were reviewed and presented in the paper. These correlations may be used to predict the

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Citation Excerpt :
Contrary to this, it also requires higher pumping power [10]. Therefore, it is recommended to use the smaller roughness elements (ribs) near the absorber plate to create turbulence near the hot plate with reduced the pumping power penalty [11]. The performance of a SAHD is influenced by three design parameters such as the shape of the air passage (rectangular, square, trapezoidal, semi-circular, and triangular), pattern or orientation of the roughness elements placed on the surface (transverse [12], inclined [13], arc [14], V-fashion [15], W-fashion [16], etc.), the number of passes of air through the duct (single-pass [17], double pass [18] and triple pass [19]), and shape of the roughness elements (circular [20], semi-circular [21], square [22], triangular [23], etc.).
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