Experimentally Analyzing the Thermal Performance of Solar Air Heater Roughened with Combined Inclined and Transverse Ribs
1
Gurjant Singh,
2Vivek Aggarwal,
3Gurpreet Singh,
4Atul Goyal
1,3Department of Mechanical Engineering, LLRIET, Moga
2Department of Mechanical Engineering, IKG Punjab Technical University (Main Campus) Kapurthala
4Professor and Principal, FCET, Ferozeshah
Email:1[email protected],2[email protected],3[email protected],
4[email protected] Abstract— This paper experimentally analyzes the thermal
performance of solar air heater roughened with combined inclined and transverse ribs. The range of parameters for this study has been decided on the basis of practical considerations of the system and operating conditions of solar air heaters. The experimental data is collected by operating open loop flow mode in actual atmospheric conditions. The data is collected for smooth duct under the same conditions in order to compare the thermal performance and to resolve the enhancement in heat transfer as a result of using roughness. The experimental investigation encompassed the Reynolds number (Re) range from 2374 to 6280; relative roughness pitch (P/e) of 8, angle of attack (α) of 60º and 90º and relative roughness height (e/Dh) is 0.047. The thermal efficiency experience 5-9%
more in roughened duct as compared to the efficiency of smooth duct.
Index Terms—Solar Air Heater, Heat Transfer, Angle of Attack, Thermal Efficiency, Reynolds Number.
I. INTRODUCTION
Energy is the foundation stone of our economic development. There are two types of energy sources:
conventional sources and non-conventional resources. As main sources of energy are fossil fuels. But these are limited so there is need of some renewable sources. The solar energy is best option for meeting energy demands.
Almost in the all fields’ solar energy fulfill the required demand, cheap as well as eco-friendly. Solar energy characterized as either active solar energy, or passive solar energy. Passive solar energy is specifically what it sounds like. Structures or buildings will be constructed in a way to capture the sun’s power through windows or tanks instead of using Panels and solar cells. But heat from sun cannot directly convert into electricity. In case of active solar energy, Solar cells and solar panels are used. They are adjusted in such a way so that they received maximum heats from sun and converted into electricity stored in batteries. Sometimes it is fed into grid system for transmission purposes.
By using artificial roughness on absorber plate, use of extended surfaces and by using packed bed solar air heater. By providing artificial roughness the laminar sub layer gets broken and it increases the heat transfer coefficient.
Varun et al. [5] performed an experimental investigation on the thermal performance having roughness elements as a combination of inclined and transverse ribs on the absorber plate in solar air heater duct for the range of Reynolds number from 2000 to 14000, relative roughness pitch from 3 to 8 as depicted in Figure 2.18. It is observed that the best thermal performance occurs for a relative roughness pitch (P/e) of 8. Kumar et al. [6] carried out an experiment on investigation of the heat transfer and fluid flow characteristics of air flow in a rectangular duct with Multi V-shaped rib with gap roughness on the heated plate. The parameters of experiments are Reynolds number (Re) range from 2000 to 20,000, relative width ratio (W/w) of 6, relative gap distance (Gd/Lv) of 0.24–0.80, relative gap width (g/e) of 0.5–1.5, relative roughness height (e/D) of 0.043, relative roughness pitch (P/e) of 10, angle of attack (α) of 60°. The maximum enhancement in Nusselt number and friction factor is observed to be 6.32–6.12 times of that of the smooth duct, respectively. The thermo-hydraulic performance parameter is found to be the best for the relative gap distance of 0.69 and the relative gap width of 1.0.V-shaped without gap Hans et al.
Singh et al. [7] performed energy based analysis of solar air heater having circular V-down rib having gap roughness on absorber plate. In this experiment the range of parameters are Reynolds number (Re) 3000-15000, Relative roughness pitch (P/e) 4 – 12, Angle of attack (α) 30° - 75°, Relative gap position (d/w) 0.2 - 0.8, Relative gap width (g/e) 0.5 – 2.0 and Relative roughness height (e/Dh) 0.015-0.043. Based on the analysis of errors in the experimental measurements for different Instruments used, the uncertainties in the values of the Reynolds
using artificial roughness but there are some studies like transverse, inclined V-shaped ribs available in which experimentation has done in actual atmospheric conditions. Combined roughness of inclined and transverse ribs has been investigated by using electric heater for temperature management. But experimental investigation has been done on flat plate solar air heater with combined inclined and transverse rib has been used for roughness on flat plate for experimental investigation.
II. EXPERIMENTAL APPARATUS AND PROCEDURE
The experimental setup consists of a test duct with entrance and exit sections, G.I. pipe, orifice plate, flow valve, flexible pipe, blower and various devices for measurement of temperature, pressure drop and solar intensity (such as thermometers, manometer and solar radiation meter respectively).
A. Galvanized iron pipe details
Experimental setup consists of 80 mm G.I pipe which supply air from duct to blower. The pipe is connected to duct on one side with conical funnel and two pieces of pipes relate to flanges having length 1000mm and 900mm. To measure the pressure difference orifice plate is placed between flanges.
B. Orifice plate
The flow rate through solar air heater is measured by orifice plate. The dimensions of G.I. pipe upstream of orifice meter are 100mm and downstream were 900mm.
The orifice diameter is 38mm and pipe diameter is 80mm.
C. Flow control valve
To regulate and change the flow rate of air, flow control valve is attached with 80mm pipe at the blower side.
D. Flexible pipe
To avoid any vibration transfer from blower to solar air heater, the flexible plastic pipe of 1500cm in length is attached to flow control valve that is leading to blower.
E. Blower unit
Blower unit consists of centrifugal blower i.e., driven by three phase electric motor of 2.2kW and 2880 r.p.m.
Centrifugal blower sucks ambient air through rectangular duct, connecting pipes and exhausts it to atmosphere.
This unit is attached with the end of flexible pipe.
The rectangular duct contains test section besides entry and exit section. The top side of test section is artificially roughened with circular inclined ribs while the other surface is smooth. Air blower sucks the atmospheric air through heated rectangular duct. The air picks up heat from the heated absorber plate and the heated air flows through the orifice plate before the blower exhausts it to atmosphere. The air flow rate through the duct is regulated by means of control valve and it is measured using orifice. Temperature measurements of air are carried out by thermometers. Manometer is used to measure the pressure drop across the test section. The rectangular duct consists of an entry section, a test section and an exit section. The duct consists of 2200 mm length;
520mm width and 25 mm depth made up of galvanized iron sheet having angle of inclination to horizontal is 45°.
The sun facing side of G.I sheet (absorber plate) is painted with dull black color to increases the absorber efficiency. To minimize the heat losses from top, 4 mm thick glass cover is fixed above the absorber plate. In exit section, the air passes through the mixing chamber having three baffles of 17 mm height and 520 mm long (equal to width of the duct) equally spaced in 75 mm length to obtain uniform exit air temperature. The cross section of rectangular duct is shown in Figure 1, whereas Figure 2 shows the direct pictures of experimental setup.
Figure 1:Schematic Diagram of Experimental Setup
Figure 2:Photograph of the Experimental Setup The roughness is produced on the Galvanized Iron sheet having the length 1900mm and width of 520 mm.
Aluminum wires are used to create artificial roughness on the underside to the G.I sheet (absorber plate). For artificial roughness, combined inclined and transverse is produced on the underside of the absorber plate.
The geometry of rib roughness can be described by the values of rib pitch (P) and rib height (e) and the parameters like relative roughness pitch (P/e), angle of attack (α) and relative roughness height (e/Dh).For this study, practical considerations of the system and operating conditions of solar air heater are used to set the range of parameters. The following table shows the value of flow and roughness parameters. The general geometry of artificially rib roughness and duct photograph of absorber plate roughened combined transverse and
Figure 3:Photographs of absorber plate roughened with combined inclined and transverse rib.
Table 1:Different Parameters and their Range
Parameter Range
Air flow rate 0.010 kg/sec-m
2– 0.029 kg/sec-m
2(R
e=2374 to 6280).
Relative roughness pitch
(P/e) 8
Angle of attack (α) 60° and 90º Relative roughness height
(e/D
h) 0.047
Experimental data for roughened and smooth plates has been collected for five different air flow rates. In
comparison and evaluation of accuracy of measurement, the flow rate for smooth rectangular duct is same under similar operation conditions on the present setup. The details of various measuring parameters are given below:
A. Mean Temperature of Air at Inlet and Outlet of the Duct
The mean temperature of air outlet (To) by weighted average method and the temperature at inlet of duct is direct measured as ambient temperature (Ta).
Ta is the ambient temperature and To is the mean temperature at outlet of the duct.
3 3 2
1 T T
T
To
B. Mass Flow Rate of Air
Mass flow rate of air has been determined from using the following relationship:
m = Q × ρ
Where m is mass flow rate of air and ρ is the air density and Q is discharge.
Mass flow rate of air is taken according to area of the absorber plate and the ranges were between 0.011kg/sec-m2to 0.027kg/sec-m2.
gha A A
A A Cd
Q 2 2
2 2 1
2 1
Where Q is a coefficient of discharge. A1is area of plate in m2, A2is area of orifice in m2and Cdis the coefficient of discharge assuming a value of 0.62.
1
a m m a h
h
Where hmis manometric height of fluid, ρmis density of manometric fluid (water 1000 kg/m2) and ρais the density of air Kg/m3.
C. Velocity of Air through Duct
The velocity of air is obtained from the calculated values of mass flow rate of air and flow area as,
airWH V m
Where m is mass flow rate (kg / sec), ρairis the density of air in Kg/m3, H is the height of the duct in m and W is the width of the duct (m).
D. Equivalent hydraulic diameter
The hydraulic diameter of the rectangular section of the duct is determined from the relationship as given below:
W
H
WH parameter
wetted
flow of area Dh
2 4 4
E. Reynolds number (Re)
The Reynolds number of air flow in the duct is calculated from the following relationship:
v VDh Re
F. Useful Gain
mcp T
a
G. Thermal Efficiency (ηth)
b Areaof
absorber plate Radiationa gain Useful
th
III. RESULTS AND DISCUSSION
Thermal efficiencies were experimentally investigated for better results and function of air flow rate is also discussed. The thermal efficiency of roughened solar air heater is higher than the smooth solar air heater due to the breakage of laminar sub-layer by the artificial rib roughened that makes the flow to turbulent. By breaking the sub layer, higher value of heat transfer from absorber plate to the flow of air. Further it has been observed that there is increase in average thermal efficiency with increase in Reynolds number. The improvement of thermal efficiency at similar flow rate conditions for smooth duct, have been compared with the roughened duct results. The variation in thermal efficiency varies from 21% to 32% in case of smooth duct whereas the value of thermal efficiency increases from 26% to 48% in roughened duct.
The thermal performances of rectangular duct with one broad wall rib-roughened by combined inclined and transverse ribs and smooth plate have been discussed.
The range of rib roughness parameters viz., relative roughness pitch (P/e) is 8, angle of attack (α) is 60° and 90º, respectively. The Reynolds number is varied from 2374 to 6280. The results obtained are presented and discussed as a function of Reynolds number in this sub-section. Figure 4 shows the change in average thermal efficiencies near about 5-12 % and the efficiency increase at every Reynolds number with using the artificial roughness. It also shows the comparison of average thermal efficiency percentage smooth duct with roughened duct. Further, it has also seen for both the ducts that there is increase in thermal efficiency with increase in Reynolds number.
1838
2374 3753 4747 5567 6280
Av er ag e th er m al ef fic ie nc y (% )
Reynolds number (Re)
Smooth Duct Roughened Duct
Figure 4:Comparison of average thermal efficiency with respect to Reynolds number between smooth and roughened.
IV. CONCLUSIONS
Thermal efficiency of combined inclined and transverse rib on absorber plate has been carried out by experimental study on solar air heater. As compare to smooth duct, roughened duct gives better results for turbulence of air.
a) It has been observed that as the value of Reynolds number increases the value of thermal efficiency also increases.
b) Combined inclined and transfer rib on the absorber plate enhances the rate of heat transfer as compared to smooth duct.
c) Comparison between smooth and roughened duct at different Reynolds number shows that average thermal efficiency increases from 5 to 12%.
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