A review of thermohydraulic performance of artificially roughened solar air heaters

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Abstract

Solar air heaters form the major component of solar energy utilization system which absorbs the incoming solar radiation, converting it into thermal energy at the absorbing surface, and transferring the energy to a fluid flowing through the collector. The efficiency of flat plate solar air heater has been found to be low because of low convective heat transfer coefficient between absorber plate and the flowing air which increases the absorber plate temperature, leading to higher heat losses to the environment resulting in low thermal efficiency of such collectors. Artificial roughness in the form of repeated ribs is the most effective and economic way of improving the thermal performance of solar air heater. This paper presents an extensive review on the research carried out on artificial roughened solar air heater ducts. The objective of this paper is to review various studies, carried out on thermal as well as hydraulic performance of artificial roughened solar air heater ducts. The review presented in this paper will be useful for the researchers working in this area.

Introduction

Energy is a basic ingredient needed to sustain life and development. Energy is needed in various forms to fulfil our day to day requirements. Energy consumption rates of the people are directly related to the prosperity or the standard of living. Two types of energy resources are available: conventional and non-conventional [1], [2]. Conventional energy resources such as fossil fuels (coal, crude oil and natural gas) are limited in amount. Total energy in recoverable conventional energy resources is estimated to be around 30–35 Q (1Q=1018 kJ) while the global energy consumption rate is roughly 0.4–0.5 Q/yr. Hence conventional energy resources are roughly estimated to last for 75–85 years. This awareness of the limited nature of conventional energy resources gave rise to the search of alternate energy resources. Non-conventional or alternate energy resources can be divided into two groups, namely, renewable and non-renewable resources. Renewable resources are those which have a short period of renewal (upto a few years) such as solar energy, wind energy, biomass energy, hydroenergy, ocean and tidal energy. Solar energy is the most promising of all these alternatives [3], [4]. Solar energy has the greatest potential among all the sources of renewable energy and even a small amount of this renewable source of energy is sufficient to meet the total energy demand of the world. If we can use 5% of this energy, it will be 50 times what the world will require. Solar energy is readily available, well distributed and inexhaustible for all practical purposes, and has no polluting effects upon the environment when converted and utilized. Solar air heaters, because of their inherent simplicity are cheap and most widely used collector devices. Solar air heaters are being used for many applications for low and moderate temperatures. Some of these are space heating, crop drying, drying of concrete and solar dryer [1], [2], [5], [6], [7].

The use of artificial roughness on the underside of the heated plate can substantially enhance the thermal performance of the solar air heater due to increase in convective heat transfer coefficient from the plate to air. Surface roughness is one of the first techniques to be considered as a means of augmenting forced convection heat transfer. In order to attain higher convective heat transfer coefficient it is desirable that the flow at the heat transfer surface should be turbulent. However, the turbulence created in the core can increase the fan power exorbitantly. It is therefore, desirable that the turbulence must be created only very close to the heat transfer surface, i.e. in the laminar sub-layer only, where the heat exchange takes place. However, as pointed out above, it is necessary that while creating turbulence to break the laminar sub-layer, the core flow should not be disturbed so as to avoid excessive losses. This can be achieved by using artificial roughness with roughness height being such that it does not project into the core but is of the height that just project out of laminar sublayer. Numbers of experimental investigations involving roughness elements of different shapes, sizes and orientations with respect to flow direction have been carried out in order to obtain an optimum arrangement of roughness geometry. Hans et al., [8] carried out a review of roughness geometry in solar air heater ducts. They discussed different roughness geometries used in solar air heater ducts and explained the concept of artificial roughness, effects of various roughness parameters on the flow pattern and also briefly discussed and reviewed the roughness geometries used 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 rate with little penalty of increase in friction losses and also discuss the thermo-hydraulic (thermal as well as hydraulic) performance of artificial roughened solar air heater ducts.

Section snippets

Artificially roughened solar air heater ducts

The application of artificial roughness in the form of fine wires or ribs having different geometries on the heat transfer surface has been recommended to increase the heat transfer coefficient by several investigators. The use of artificial roughness on the underside of the absorber plate disturbs the viscous sub-layer of the flowing medium. It is well known that in a turbulent flow a sub-layer exists in the flow in addition to the turbulent core. The purpose of the artificial roughness is to

Types of artificial roughness elements investigated

Prasad and Mullick [9] studied the effect of protruding wires on friction factor, heat transfer rate and plate efficiency factor of a solar air heater used for drying of agricultural products. High mass flow rates were used to have turbulent flow in the ducts and protruding wires of 1 mm diameter tripped the laminar sub-layer. They compared the heat transfer, friction factor and plate efficiency factor of roughened corrugated and roughened plane absorber plates with that of corrugated without

Heat transfer and friction factor correlations

In most of the solar air heaters roughened artificially for heat transfer enhancement, correlations have been developed for heat transfer coefficient and friction factor in terms of roughness geometry parameters. Some of the correlations developed for heat transfer coefficient and friction factor are given in Table 2.

Thermal performance of solar air heater

It was pointed out earlier that the low thermal efficiency of solar air heaters can be improved by using artificial roughness in the form of different shapes fabricated in various arrangements to create turbulence near the wall or to break the viscous sub layer. As a result, increasing the heat transfer coefficient, thermal efficiency can be increased but at the same time creating turbulence requires additional energy which has to be supplied by fan or blower at the expense of electrical energy.

Thermohydraulic performance of roughened solar air heaters

Studies have been conducted to develop methodologies for assessing the net heat gain in energy due to artificial roughness as augmentation of heat transfer vis-à-vis enhancement of pumping power requirement. Efforts have been made to optimize such a system for a required temperature rise if the heat transfer and the friction factor data are available. The existence of optimum condition for a given rough surface was proposed by Sheriff and Gumley [38]. It was shown that the characteristics of a

Comparison effective efficiency of roughened solar air heaters duct

The solar air heater having artificially roughened absorber plate have higher heat transfer coefficient, through accompanied by an increase in friction factor. Correlations for Nusselt number and friction factor have been developed in terms of roughness and flow parameters by various investigators. A comparison of performance of roughened solar air heaters with that of conventional solar air heater s having different type roughness geometries namely here transverse continuous rib, inclined or

Conclusions

  • (i)

    The literature review shows that the use of artificial roughness in different forms and shapes is an effective and economic way of improving the performance of solar air heaters.

  • (ii)

    Numbers of experimental investigations involving roughness elements of different shapes, sizes and orientations with respect to flow direction have been carried out in order to obtain an optimum arrangement of roughness element geometry.

  • (iii)

    Correlations for heat transfer and friction factor have been developed which are

References (61)

  • R. Karwa et al.

    Heat transfer coefficient and friction factor correlations for the transitional flow regime in rib roughened rectangular ducts

    Int J Heat Mass Transf

    (1999)
  • R.P. Saini et al.

    Heat transfer and friction factor correlations for artificially roughened ducts with expanded metal mesh as roughened element

    Int J Heat Mass Transf

    (1997)
  • S.K. Saini et al.

    Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having arc-shaped wire as artificial roughness

    Sol Energy

    (2008)
  • S.V. Karmare et al.

    Heat transfer and friction factor correlation for artificially roughened duct with metal grit ribs

    Int J Heat Mass Transf

    (2007)
  • J.L Bhagoria et al.

    Heat transfer coefficient and friction factor correlations for rectangular solar air heater duct having transverse wedge shaped rib roughness on the absorber plate

    Renew Energy

    (2002)
  • R.P. Saini et al.

    Heat transfer and friction factor correlations for a duct having dimple-shaped artificial roughness for solar air heaters

    Energy

    (2008)
  • A. Lanjewar et al.

    Experimental study of augmented heat transfer and friction in solar air heater with different orientations of w-rib roughness

    Exp Thermal Fluid Sci

    (2011)
  • A. Layek et al.

    Heat transfer coefficient and friction characteristics of rectangular solar air heater duct using rib-grooved artificial roughness

    Int J Heat Mass Transf

    (2007)
  • A.R. Jaurker et al.

    Heat transfer and friction characteristics of rectangular solar air heater duct using rib-grooved artificial roughness

    Sol Energy

    (2006)
  • V.S. Hans et al.

    Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with multiple v-ribs

    Sol Energy

    (2010)
  • P. Biondi et al.

    Performance analysis of solar air heaters of conventional design

    Sol Energy

    (1988)
  • S.V. Karmare et al.

    Experimental investigation of optimum thermohydraulic performance of solar air heaters with metal rib grits roughness

    Sol Energy

    (2009)
  • N. Sheriff et al.

    Heat transfer and friction properties of surfaces with discrete roughnesses

    Int J Heat and Mass Transf

    (1966)
  • R.L. Webb et al.

    Heat transfer and friction in tubes with repeated-rib roughness

    Int J Heat Mass Transf

    (1971)
  • M.J. Lewis

    Optimizing the thermohydraulic performance of rough surfaces

    Int J Heat and Mass Transf

    (1975)
  • A. Cortes et al.

    Improvement of efficiency of a bare solar collector by means of turbulence promoters

    Appl Energy

    (1990)
  • R. Karwa et al.

    Thermo-hydraulic performance of solar air heaters having integral chamfered rib roughness on absorber plates

    Energy

    (2001)
  • M.K. Mittal et al.

    Effective efficiency of solar air heaters having different types of roughness elements on the absorber plate

    Energy

    (2007)
  • K. Altfeld et al.

    Second Law optimization of flat plate solar air heaters

    Sol Energy

    (1988)
  • P. Naphon

    On the performance and entropy generation of the double pass solar air heater with longitudinal fins

    Renew Energy

    (2005)
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