Experimental investigation of thermal efficiency of solar air heater duct with different spacing ribs as artificial roughness

Experimental investigation of thermal efficiency of solar air heater duct with different spacing ribs as artificial roughness

Aditya Takhalate1 Dr.A. R. Jaurker2

1 M.E. Student Heat Power Engineering Department of Mechanical Engineering

2 Professor Department of Mechanical Engineering Jabalpur Engineering College Jabalpur

Keywords: artificial roughness, solar air heater, thermal efficiency, nusselt number

no polluting effect on the environment when converted in useful forms and utilized. However, its availability is intermittent, i.e. only available during the day. It is also dependent on the climatic and weather conditions. Some of the important applications of solar energy are solar cooking Solar water heating,solar space heating and cooling, solar crop drying, solar power generation. In order to make the solar energy utilization economically viable, one of the important requirements is its efficient collection. Flat plate collector is the most important type of solar collector because it is simple in design, cheap in construction, easy in operation and little maintenance cost. It is designed for a variety of applications in which temperature ranging from ambient to 100ºC. [7]The principle usually followed is to expose a dark surface to solar radiation so that radiation is absorbed. Apart of the absorbed radiation is then transferred to fluid like air or water. But the efficiency of solar air heater is usually low due to lower value of heat transfer coefficient. Thus artificial roughness is provided on lower side of absorber plate to increase the value of heat transfer coefficient by creating turbulence in rectangular duct.

The artificial roughness has been used extensively for the enhancement of heat transfer and thermal efficiency of solar air heater. The use of artificial roughness in solar air heaters owes its origin to several investigations carried-out in connection with the enhancement of heat transfer in nuclear reactors cooling of turbine blades and electronic components. For basic type of roughness geometry, the key dimensionless variables are as follows:

1. Relative roughness height (e/Dh) defined as the ratio of roughness height to hydraulic diameter of duct.

2. Relative roughness pitch (p/e) defined as the ratio of the distance of two successive roughness elements to the height of the roughness.

3. Angle of attack ( ) defined as angle of roughness elements with respect to the flow direction.

1. Several investigators have analyzed of thermal performance of flat plate solar air heaters. The thermal performance of the solar air heater is described by an energy balance, which indicates the distribution of incident solar energy into useful energy gain (Qu) and various losses (QL) as shown in Fig.1. The detail of the performance analysis of a solar collector is given below [4].Energy balance on the collector can be written as:

   Ac [I R )] = Qu + QL (1)

   The useful energy Qu, can be written in the terms of known temperature Ti and other system and operating parameters as

   Qu Ac FR S

   U L Ti Ta

   (2)

   The thermal efficiency of a collector can be written as

   th FR

   Ti Ta

   U

   L I

   (3)

   The heat removal factor can be expressed as:

   FR GCP 1 UL

   exp

   FPUL GCP

   (4)

   Where Fp is the collector efficiency factor defined as the ratio of actual useful heat collection rate to that attainable with entire absorber plate at average fluid temperature. The collector efficiency factor for solar air heater can be expressed as:

   F h

   p h UL

   (5)

   A parameter F, known as flow factor is defined as follows

   GCP 1

   F U L Fp

   exp

   FPU L

   GCP

   (6)

   Knowing the value of factors FR , F and other parameters the mean fluid temperature ( Tf ) and

   _

   the mean plate temperature Tp

   can be calculated from the following equations,

   _ Qu / Ac

   Tf Ti

   UL FR

   1 F"

   (7)

   _ Qu / Ac

   Tp Ti

   UL FR

   1 FR

   (8)

   The air heater performance was computed for the most useful range of designs and working conditions. In particular condition of a solar air heater drawing air from the atmosphere, the inlet air temperature coincides with the ambient and the Eq. (3) then reduces to,

   th FR

   (9)

   This expression of efficiency does not allow the real operative temperature to be shown. In view of these limitations, [2] proposed the following equation for the efficiency of the solar air heater,

   th Fo

   To Ti

   U

   L I

   (10)

   where, Fo is the heat removal factor referred to outlet air temperature and is expressed as:

   F GCP

   exp

   FPU L 1

   U

   G

   C

   o (11)

   L P

   Further, performance can also be expressed by another equation, containing temperature gain by the fluid flowing through the collector as given below,

   th GC p

   To Ti I

   (12)

2. 1. Reynolds number, (Re) = 2000 to 14000

   2. Relative roughness pitch,(p/e) =6-21

   3. Relative roughness pitch,(e/Dh)=0.036

   4. Angle of attack=90°

3. An experimental setupas recommended in ASHRAE Standard 93-77(1977) [1] is shown in Fig.2 consists of wooden rectangular duct. The wooden rectangular duct of internal size 2100 mm X 200 mm X 25 mm includes entrance section, test section and exit sections mixing, length of 400 mm, 1500 mm, and 200 mm respectively. An electrical heater of size 1500 mm X 200 mmwas fabricated by nichrome wire as desired requirement.The calibrated copper constantan 28 SWG thermocouples were used to measure the air and the heated plate temperature at different locations, as shown in Fig.4.A thermos flask is used to maintain temperature of ice water mixture. A digital multimeter is used to indicate the output of the thermocouples through the selector switch. The air flow is measured by means of orifice meter fitted in pipe line and sucked through the rectangular duct by means of a blower driven by a 3-phase 440V, 2.3kW and 1420 rpm, A.C. motor. Before starting the experimental all the thermocouples were checked carefully so that they give the room temperature and all the pressure tapings were checked for the leakage problem.

   	Figure 2 Experimental Set Up
   1.Entrance Length		6.Orifice Plate
   2.Test Length		7.U Tube Manometer
   3.Exit Section		8.Control Valve
   4.Transition Section		9.Blower
   5. GI Pipe		10.Flexible pipe 300mm

4. The test runs to collect relevant heat transfer were conducted under quasi-steady state conditions. The quasi-steady state condition was assumed have to been reached when the temperature at the point does not change for about 10 minutes. When a change in the operating conditions is made, it takes about 30 min to reach such a quasi-steady state. Six values of flow rates were used for each set at a fixed heat flux of the test. After each of flow rate, the system is allowed to attain a steady state before the data were recorded.[5, 6, 7, 8]

   The following parameters were measured:

   1. Temperature of heated plate

   2. Temperature of air at inlet and outlet of the test section

   3. Pressure difference across orifice meter

5. Te nusselt number evaluated from experimental data (Fig.5) for smooth duct have been compared with that of smooth duct have been compared with the values obtained from dittus boelter equation as:

   Nus = 0.023 Re0.8 Pr 0.4

   The above equation is used to estimate the nusselt number at different Reynolds number and compared with experimental measured values.

   Dittus boelter equation

   experimental predicted

   45

   40

   35

   Nusselt number

   30

   25

   20

   15

   10

   5

   0

   0 5000 10000 15000

   Reynolds number

6. The effect of various flow and roughness parameters on heat transfer characteristics in rectangular duct has been shown below. In each graph the results of smooth duct have been compared with that of roughened duct under similar operating conditions in order to determine the enhancement in heat transfer.

   smooth p/e=6 p/e=12 p/e=18 p/e=21

   120

   100

   Nusselt number

   80

   60

   40

   20

   0

   0 2000 4000 6000 8000 10000 12000 14000 16000

   Reynolds number

   smooth p/e=6 p/e=12 p/e=18 p/e=21

   0.9

   0.8

   Thermal Efficiency

   0.7

   0.6

   0.5

   0.4

   0.3

   0.2

   0.1

   0

   0 0.005 0.01 0.015 0.02

   (To-Ti)/I

   To-Ti/I=0.004 To-Ti/I=0.006 To-Ti/I=0.008 To-Ti/I=0.01

   120

   100

   thermal

   80

   60

   40

   20

   0

   0 5000 10000 15000

   Reynolds number

   Vol. 1 I9ssue 5, July – 2012

   smooth p/e=6 p/e=12 p/e=18 p/e=21

   0.9

   0.8

   0.7

   0.6

   thermal

   0.5

   0.4

   0.3

   0.2

   0.1

   0

   0 5000 10000 15000

   Reynolds Number

   Figure 6 shows the variation of Nusselt number with Reynolds number. The nusselt number increases with increase in Reynolds number because the value of heat transfer coefficient increases as the value of Reynolds number approaches from laminar to turbulent region due to decrease in convective resistance.It is also noticed that at lower Reynolds number (Re < 5000) the roughness has negligible effect on nusselt number and the results are nearly equal to that of smooth duct.It is so because the rib retarded the fluid flow thus laminar sub layer will be formed and the laminar sub layer remains unbreakable which offers resistance to fluid flow and hence lower value of heat transfer coefficient. Figure 7 shows the effect of temperature rise parameter on thermal efficiency at different values of relative roughness pitch.As the value of temperature rise parameter increases the efficiency increases. Figure 8 shows the effect of temperature rise parameter at different Reynolds number. Thermal efficiency decreases at lower value of temperature rise parameter.Figure 9 shows the values of thermal efficiency at different relative roughness pitch and compared with those of smooth duct. And it was found that the maximum value of thermal efficiency was 83% at a relative roughness pitch of 12.

From the experimental results of heat transfer for rectangular duct with transverse staggered discrete ribs in one broad wall subjected to constant heat flux, the main findings are:

1. Heat transfer and thermal performance of solar air heater ducthave higher values than that of the smooth solar heater.

2. The nusselt number for the roughened rib is2.26 times higher as compared to smooth rectangular duct under similar conditions.

3. It was found that the maximum enhancement inefficiency is 2.07 times higher than that of smooth rectangular duct.

Ac area of absorber plate, m2

Cp specific heat at constant pressure, J/Kg K

Dh equivalent or hydraulic Diameter of duct, m

FR Heat removal factor

1. mass flow rate per unit area of collector, kg/sm2

   h heat transfercoefficient W/m2K

2. height of air duct,m

I,S Intensity of solar radiations,W/m2

K Thermal conductivity of air,W/mK

Qu Useful heat gain, W

T b bottom plate temperature,K

To outlet air temperature, K

Ti inlet air temperature, K

Ta ambient temperature, K

UL overall heat loss coefficient/m2K

Tf bulk mean air temperature,K

v velocity of fluid, m/s

W width of duct,m

Dimensionless parameters

e/Dh relative roughness height

Nu Nusselt number

Pr Prandtl number

p/e relative roughness pitch

Re Reynolds number

Greek Symbols

transmittance absorptance product

angle of attack

thermal thermal efficiency

1. ASHARE Standard 93-97. Method of testing to determine the thermal performance of solar collectors, 1977

2. Biondi, P. and Cicala, L and Farina, G, Performance analysis of solar air heaters of conventional design, Solar Energy, 41,101-107,1988.

3. ChoudhuryC,Andersen S and Rekstad J,A solar air heater for low temperature application, solar energy, 40,335-343, 1988.

4. Duffie, J.A. and Beckman, W.A., Solar engineering of thermal processes, Wiley, New York, 1980.

5. M.M. Sahu, J.L. Bhagoria, Augmentation of heat transfer coefficient by using 90° broken transverse ribs on absorber plate of solar air heater, Renewable Energy 30 (2005) 2057 2073.

6. Lau SC, McMillan RD, Han JC. J Turbo machinery Trans ASME 1991; 113:3606.

7. Sukhatme SP. Solar Energy. 4th: Tata McGraw Hill Publishing Company Limited; 1999. New Delhi.

8. Varun, R.P. Saini, S.K. Singal. Investigation of thermal performance of solar air heater having roughness elements as a combination of inclined and transverse ribs on the absorber plate, Renewable Energy 33 (2008) 13981405.