View of A CFD ANALYSIS OF AN ARTIFICIALLY ROUGHENED SOLAR AIR HEATER

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Vol. 05,Special Issue 01

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(ICOSD-2020) January 2020, Available Online:

www.ajeee.co.in/index.php/AJEEE A CFD ANALYSIS OF AN ARTIFICIALLY ROUGHENED SOLAR AIR HEATER

1

Mayank Kumar Dwivedi,

²

Kundan Gupta,

³

Krishna Dutta Pandey,

4

Sanjay Kumar Patel

1,2,3

Assistant Professor, Mechanical Engineering Department, Lakhsmi Narain College of Technology, Indore, MP

4

Assistant Professor, Mechanical Engineering Department, Lakhsmi Narain College of Technology & Science, Indore, MP

Abstract- This paper presents the study of fluid flow and heat transfer in a plain rectangular duct of a solar air heater by using Computational Fluid Dynamics (CFD). The effect of Reynolds number on heat transfer coefficient and friction factor is investigated. A commercial finite volume package ANSYS FLUENT 12.1 is used to analyze and visualize the nature of the flow across the duct of a solar air heater. CFD simulation results are found to be in good agreement with experimental results and with the standard theoretical approaches. It has been found that the Nusselt number increases with increase in Reynolds number and friction factor decreases with increase in Reynolds number.

Keywords: Energy, Solar Air Heater, Heat transfer, Pressure Drop, CFD.

1. INTRODUCTION

Solar air heater is one of the basic equipment through which solar energy is converted into thermal energy. Solar collectors in the system for the utilization of solar thermal energy are widely used in various equipment. Solar collectors (air heaters), because of their simple in design, are cheap and most widely used collection devices of solar energy. The main application of solar air heaters are space heating, seasoning of timber, curing of industrial products and these can also be effectively used for curing/drying of concrete/clay building components. A conventional solar air heater generally consists of an absorber plate with a parallel plate below forming a small passage through which the air is to be heated and flows as shown in Fig. 1. A solar air heater is simple in design and requires little maintenance.

2. THERMAL PERFORMANCE OF SOLAR AIR HEATER

Performance of any system represents the degree of utilization of input to the system. It is

required to analyze thermal and hydraulic performance of a solar air heater for making an

efficient design of such type of a system. Thermal performance concerns with heat transfer

process within the collector and hydraulic performance concerns with pressure drop in the

duct. A conventional solar air heater (Fig. 1) is considered for brief analysis of thermal and

hydraulic performance in the following sub-sections.

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www.ajeee.co.in/index.php/AJEEE 2.1 Thermal performance

Thermal performance of solar air heater can be expressed in terms of the fraction of incident solar radiation utilized to increase the temperature of air. In other words, Thermal efficiency is a measure of thermal performance of a solar air heater. Thermal performance of solar air heater can be computed with the help of Hottel–Whillier–Bliss equation reported by Duffie and Beckman [9].

or

The rate of valuable energy gain by flowing air in the course of duct of a solar air heater can be intended as follows equation:

The value of heat transfer coefficient (h) can be increased by various active and passive augmentation techniques. It can be represented in non-dimensional form of Nusselt number (Nu).

Further, thermal efficiency of a solar air heater can be expressed by the following equation;

2.2 Hydraulic performance

Hydraulic performance of a solar air heater concerns with pressure drop (ΔP) in the duct.

Pressure drop accounts for energy consumption by blower to propel air through the duct.

The pressure drop for fully developed turbulent flow through duct with Re< 50, 000 is given as

3. COMPUTATIONAL DOMAIN

The 2-D computational domain used for CFD analysis having the height (H) of 50 mm and width (W) 50 mm and length of 500 mm as shown in Fig. 2 and Fig 3.

Fig. 2. 2-D computational domain.

Fig. 3. 2-D computational domain with semicircular rib.

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www.ajeee.co.in/index.php/AJEEE 3.1 Parameters for computational analysis

4. CFD ANALYSIS

In the present study, FLUENT Version 12.1 is used for analysis.

The assumptions were made in the mathematical model during CFD analysis are:- 1. The flow is fully developed, steady, turbulent and three dimensional.

2. The thermal conductivity does not change with temperature.

3. The working fluid, air is assumed to be incompressible.

4.1 Mesh Generation

After defining the computational domain, non-uniform mesh is generated. In creating this mesh, it is desirable to have more cells near the plate because we want to resolve the turbulent boundary layer, which is very thin compared to the height of the flow field.

Fig. 4. Close up view of the two-dimensional non-uniform mesh 4.2 Specify Boundary condition

This will bring up the specify boundary types window on the operation panel. We will first specify that the left edge is the duct inlet and right edge is the duct outlet. Top edge is top surface and bottom edges are inlet length, outlet length and solar plate. All internal edges of rectangle 2D duct are defined as turbulator wall.

Edge Position Name Type

Left Duct Inlet VELOCITY_INLET

Right Duct

Outlet PRESSURE_OUTLE T

Top Top

Surface WALL Bottom edge-1 Inlet

Length WALL Bottom edge-2 Solar Plate WALL Bottom edge-3 Outlet

Length WALL

Internal Edges Turbulator WALL

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www.ajeee.co.in/index.php/AJEEE 4.3 Solver

FLUENT Version 12.1 is used as a solver with k-epsilon turbulence model.

5. RESULTS AND DISCUSSION

Fig 5 shows the temperature contour for the semicircular shape of ribs inserted in a solar air heater duct.

Fig. 5. Temperature Contour

Fig 6 shows the contour of stream function for the semicircular shape of ribs inserted in a solar air heater duct. The heat transfer phenomenon can be observed and described by the temperature contour and contour of stream function.

Fig. 6. Contour of stream function

CFD analysis predicts performance of various ribs at different p/e which is demonstrated through the figure given below. It can be seen (Fig 7) that there is a substantial enhancement caused by providing artificial roughness in the form of turbulator roughness.

The enhancement in Nusselt number compared to that of the smooth surface achieved

varies from 2.5 to 3.5 times for the entire data collected.

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Fig. 7. Variation of Nu with Re

Fig. 8 shows the predicted results of friction factor using the k-ε turbulence model for different p/e in the range of Reynolds number investigated for fixed values of other parameters. It is seen that the friction factor decreases with increasing Reynolds number in all cases due to the suppression of viscous sub-layer with the increase of Reynolds number.

Fig. 8. Variation of friction factor with Re 6. CONCLUSIONS

A CFD analysis has been carried out to predict the heat transfer and fluid flow characteristics of a solar air heater having roughened duct provided with semicircular rib roughness.

The following conclusion can be drawn from this study:-

1. Nusselt number increases with the increase of Reynolds number.

2. Friction factor decreases with the increase of Reynolds number.

3. Solar air heater with semicircular rib roughness gives 2.5 to 3.5 times enhancement in Nusselt number as compared to smooth duct.

4. Solar air heater with semicircular rib roughness gives 2 to 3 times enhancement in Friction factor as compared to smooth duct.

5. The maximum value of Nusselt number has been found corresponding to relative

roughness pitch of 14.

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www.ajeee.co.in/index.php/AJEEE 6. The maximum value of Friction factor has been found corresponding to relative

roughness pitch of 10.

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