PERFORMANCE ANALYSIS FOR V-GROOVE ABSORBER · Hottel-Whillier-Bliss equation (Duffie and Beckman,...
Transcript of PERFORMANCE ANALYSIS FOR V-GROOVE ABSORBER · Hottel-Whillier-Bliss equation (Duffie and Beckman,...
PERFORMANCE ANALYSIS FOR V-GROOVE ABSORBER
Introduction
Solar air heaters form the major component ofsolar energy utilization systems which absorbthe incoming solar radiation, converting it intothermal energy at the absorbing surface, andtransferring the energy to a fluid flowing throughthe collector. Solar air heaters, because of theirinherent simplicity, are cheap and the mostwidely used collection device (Momin et al.,2002). It has a wide range of applications indrying agricultural products, industrial processheating in textile and papers, space heating andgreenhouse heating. Several designs of solar aircollectors have been studied over the years,hence the design of suitable air collectors is oneof the most important factors controlling the
economics of solar drying. Therefore extensiveinvestigations have been carried out on theoptimum design of conventional and modifiedsolar air heaters, in order to search for efficientand inexpensive designs suitable for massproduction for different practical applications.Previous researchers have focused on the effectsof design and operational parameters, type offlow passes, number of glazing and types ofabsorbers flat, corrugated or finned, on thethermal performance of solar air heaters (Vermaet al., 1991; Verma et al., 1992; Choudhuryet al., 1995 and Karim et al., 2004). SopianSupranto et al. (1999) studied the use of porousmedia in doublepass solar air heaters and
Department of Mechanical and Manufacturing Engineering, Faculty of Engineering University PutraMalaysia, 43400 UPM Serdang, Selangor, Malaysia. Tel.: 0060-126771656; E-mail: [email protected]
* Corresponding authorSuranaree J. Sci. Technol. 14(1):39-52
Bashria Abdrub alrasoul Abdallah Yousef* and Nor Mariha Adam
Received: Mar 31, 2006; Revised: Nov 8, 2006; Accepted: Nov 11, 2006
Abstract
This paper concerns theoretical study to investigate the effect of mass flow rate, flow channel depthand collector length on the system thermal performance and pressure drop through the collector, onV-groove absorber at single and double flow mode. This study has been conducted by using a developedinternet based mathematical simulation. It concludes that in V-groove absorbers types, the double flowmode is 4 - 5% more efficient than the single mode. On the other hand the use of porous media in doubleflow increases the air heater efficiency to be 7% more efficient than an air heater in single mode, and2 - 3% more efficient in double flow mode without porous media. The analysis and the graphs obtainedin this paper can be a helpful tool for a design engineer to construct economical and efficient solar airheaters with technical dimensions.
Keywords: Single and double flow V-groove absorber, porous media, thermal performance, pressure
drop, flow channel depth
40 Performance Analysis for V-groove Absorber
found that the use of porous media in the secondchannel of solar air heaters increases the heattransfer area. In addition, this type of air heaterhas a higher thermal performance comparedto the conventional single-pass solar air heater.An internet based mathematical simulationhas been conducted and developed by Bashriaet al. (2004a, 2004b) to predict the thermalperformance for different designs of solar airheaters with flat or V-groove absorbers, single,double or triple glass covers, in single passor double pass double ducts with or withoutporous media.
This study uses the aforementioneddeveloped program by Bashria et al. (2004a,2004b) to find the effect of different parameters,such as mass flow rate, flow channel depthand collector length on the system thermalperformance and pressure drop through thecollector, for V-groove absorbers in single anddouble passes with and without using a porousmedia.
Theoretical Analysis
To model the solar air types and obtain theirrelative equations, a number of simplifyingassumptions has been made to lay thefoundation without obscuring the basic physicalsituation. These assumptions are as follows:
1. Performance is steady state.2. There is no absorption of solar energy
by a cover insofar as it affects lossesfrom the collector.
3. Heat transfer fluid is considered anon-participating medium.
4. The radiation coefficient between thetwo air duct surfaces is found byassuming a mean radiant temperatureequal to the mean fluid temperature.
5. Loss through front and back are to thesame ambient temperature.
In single pass V-groove absorber,energy is transferred from the plate to thebottom of the cover through both the radiationheat transfer coefficient h
r and the convection
heat transfer coefficient h1, and to the fluid at a
temperature at Tf through the convection heat
transfer coefficient h2. Energy is transferred from
the fluid to the ambient air at Ta through the
back loss coefficient Ub. Energy is lost to the
ambient air through the combined convectionand radiation coefficient U
t. Schematic
drawings for single pass V-groove absorberand thermal net work are shown in Figure 1(a).The steady-state energy equations yield thefollowing equations (Bashria et al., 2004a):
Collector cover
h1 (Tp -Tc )+hr (Tp -Tc ) = Ut (Tc -Ta ) (1)
V-groove absorber
h2 (Tp -Tf )+h1 (Tp -Tc ) + hr (Tp -Tc ) =Ιτα (2)
Fluid medium
h2 (Tp -Tf )+Ub (Ta -Tf ) = (______) (______) (3)
In double pass double duct energy istransferred from the plate at T
P to the cover
through the radiation heat transfer coefficienth
r1 and the convection heat transfer h
1, and to
the fluid in the upper duct flowing throughthe V-groove at a temperature T
f1 through
the convection heat transfer coefficient h2, and
to the fluid in the lower duct flowing betweenthe absorber plate and the base plate at atemperature T
f2 through the convection heat
transfer coefficient h3. Also energy is transferred
to the base plate through the radiation heattransfer coefficient h
r2. Energy is transferred from
the fluid flowing in the lower duct at Tf2 to the
base plate through convection heat transfercoefficient h
4. Finally, energy is lost to the
ambient air through the combined convectionand radiation coefficient U
t through the cover
glass. Figure 1(b) illustrates the double passdouble duct V-groove absorber and the thermalnet work. The steady-state energy balance onthe cover, the plate and the fluid in the upperand lower ducts (with the assumption thatthe heat transfer fluid is a non participatingmedium) gives the following equations:
mCp
W
dT f
dx
41Suranaree J. Sci. Technol. Vol. 14 No. 1; January - March 2007
Fluid medium in the upper passage
h2 (Tp -Tf1 ) = (_______) (_______) (11)
Fluid medium in the lower duct
h3 (Tp -Tf2 ) + h4 (Tpr -Tf2 ) +
h5 (Tf2 -Tr ) = (_______) (_______) (12)
Porous media
hr2 (Tp -Tpr ) + h6 (Tpr -Tr ) = h4 (Tpr -Tf2 ) (13)
Bottom plate
h5 (Tf2 -Tr ) + h6 (Tpr -Tr ) = Ub (Tr -Ta ) (14)
The thermal efficiency which is definedas the ratio of the useful energy to the totalincident solar radiation is expressed by theHottel-Whillier-Bliss equation (Duffie andBeckman, 1991).
(15)
Heat Transfer Coefficients
In order to solve the models, the convectiveheat transfer coefficient for air flowing overthe outside surface of the top glass cover andinside channel are needed. The followingcorrelation proposed by McAdams (1954)for air flowing over the outside surface of theglass cover is used to predict the convective heattransfer coefficient
ha = 5.7 + 3.8V (16)
where ha is the convective heat transfercoefficient, and V is the wind velocity.
The radiation heat transfer coefficientfrom the absorber to the glass cover can be statedas follows using Duffie and Beckman, (1991)and Verma et al. (1992)
Collector cover
h1 (Tp -Tc )+hr (Tp -Tc ) = Ut (Tc -Ta ) (4)
V-groove absorber
h2 (Tp -Tf1 ) + h1 (Tp -Tc ) + hr (Tp -Tc )
+ h3 (Tp -Tf2 ) + hr2 (Tp -Tr )= Ιτα (5)
Fluid medium in the upper passage
h2 (Tp -Tf1 ) = (_______) (_______) (6)
Fluid medium in the lower duct
h3 (Tp -Tf2 ) = (_______) (_______) + h4 (Tf2 -Tr ) (7)
Bottom plate
h4 (Tf2 -Tr ) + hr2 (Tp -Tr ) = Ub (Tr -Ta ) (8)
In case of double flow double duct packedwith porous media (glass-wool of 0.8 porosity)energy is transferred from the plate at Tp to theporous media through the radiation heat transfercoefficient h
r2. Energy is transferred from the
porous media to the fluid in the lower ductthrough the convective heat transfer coefficienth
4 and from the fluid at T
f2 to the bottom plate
through convection heat transfer coefficienth
5.
Also energy is transferred from the
porous media to the bottom plate through theconvection heat transfer coefficient heattransfer coefficient h
6. Figure 1(c) illustrates
the double pass double duct V-groove absorberwith porous media in the lower duct and thethermal net work. The steady-state energybalance gives the following equations
Collector cover
h1 (Tp -Tc ) + hr (Tp -Tc ) = Ut (Tc -Ta ) (9)
V-groove absorber
h2 (Tp -Tf1 ) + h1 (Tp -Tc ) +hr (Tp -Tc ) +
h3 (Tp -Tf2 )+ hr2 (Tp -Tpr) = Ιτα (10)
mCp2
W
dT f2
dx
mCp1
W
dT f1
dx
mCp2
W
dT f2
dx
mCp1
W
dT f1
dx
42 Performance Analysis for V-groove Absorber
Ta
Tc
1/Ut
1/h1
1/h2Tf qu
1/hr
1/Ub
TpI
Tc
Tph1
Insulation
h2
UbFluid flow
Tf
CoverUtI
1/h41/hr2
1/h3
1/h2
Tp
Tc
1/h1
1/Ut
1/hr1
I
Ta
Tf1 qu1
Tf2 qu2
Tr
Cover Ut
hr1
Fluid inlet
h2h1
Tp Ub
h4
Ub Insulation
h3
hr2
Tf1
I
Fluid outlet Tf2
I
Cover Ut
hr1
h2
h6
Ub Insulation
Fluid inlet
Tr1 h1 Tp
h4
h3 h5hr2
Tr
I
Floid outlet T2 1/hr2
1/hr1 1/h1
1/Ut
1/Ub
Tpr
1/h6 1/h5
1/h4
1/h3
1/h2
Tp
Tc
Ta
Tf2
Tf1 qu1
qu2
Tr
Figure 1. Schematic diagram of V-groove solar air heaters with thermal network
43Suranaree J. Sci. Technol. Vol. 14 No. 1; January - March 2007
(17)
In the V-groove the radiation is reflectedseveral times in the grooves, each timeabsorbing a fraction of the beam. This multipleabsorption gives an increase in the solarabsorptance but at the same time increases thelong-wavelength emittance (Ruslan et al., 2003).
(18)
The convective heat transfer coefficientsare calculated using the following relations
(19)
where Nu is Nusselt number, k is the air thermalconductivity and D
h is the equivalence diameter
of the channel. Nusselt number for laminar flowregion (Re < 2300), transition flow region (2300< Re < 6000), and turbulent flow regionrespectively as mentioned by (Naphon, 2005) are:
(20)
Nu = 0.116(Re _125) Pr 2 / 3 1 / 3
(1+ (___) 2 / 3
)(___) 0.14Dh
L w
(21)
(22)
where Re is the Reynolds number, Pr is PrandtlNumber
(23)
Dh = 4 _________________________ (24)
For the V-groove absorber it is found thatthe hydraulic diameter equals (Bashria et al.,2004a):
Dh = _____________ (25)
Pressure Drop
When the air flows through the channel inthe air heater, due to friction the air pressuredrops along the length of the flow channel. Thispressure drop across the flow duct is given bythe following expression (Verma et al., 1992):
(26)
(27)
The values of fo and y are
fo = 24 / Re, y = 0.9for laminar flow (Re < 2550)
fo = 0.0094, y = 2.92Re -0.15
for transitional flow (2550 < Re < 104)fo = 0.059Re-0.2, y = 0.73
for turbulent flow (104 < Re < 105)
Simulation Procedure
A prototype internet-based computer programwas developed to be used as a tool to supportthe design of solar air heaters used for dryingagricultural products in Malaysia. Dream-weaverTM combined with the Active Server Pages
Cross sectional area of the flowwetted perimeter
2Hv sin (φ / 2)1 + sin (φ / 2)
44 Performance Analysis for V-groove Absorber
(ASP) as an extension environment, have beenchosen as a development software tool. Theprogrammatic code was written by VBScript asscripting language to execute commands on acomputer (Bashria et al., 2004a). Rules havebeen used to specify a set of actions performedfor a given situation. The required data areclassified into three groups, the general inputdata concerning metrological conditions, thecollector characteristics data that contain allspecific manufacturing attributes related to thecollector and the energy characteristics datawhich contain measured data about inlettemperature and mass flow rate related to thetransfer media inside the collector (Bashriaet al., 2004b). The aforementioned internet basedcomputer program is in the process of continualdevelopment but it can be accessed through thewebsite http://www.eng.upm.edu.my/home/bashria/public_html/default.asp
Initial work for simulation was based ondetermining the types and components ofsolar air heaters, its use, advantages anddisadvantages. In developing the numericalprocedure, several steps and processes areinvolved. The primary task can be categorizedinto five steps, as follows: Identification andcharacterization of the problem, conceptualiza-tion, mathematical model, implementation andtesting. The flow cart of such developmentprocesses is shown in Figure 2. The numericalalgorithm and mathematical model that used tocalculate the thermal performance were basedon Hottel-Wrillier-Bliss equation and on thedeveloped energy balance equations at variouscomponents of the collector models, along withthe different heat transfer coefficients at theirsurfaces which have mentioned previously in thetheoretical analysis section. These equations havebeen transferred to syntax code and implantedinto the developed computer program.
Numerical values of different para-meters such as outlet temperature, efficiencyand the pressure drop were computedcorresponding to an ambient temperature 33oC,solar radiation 500 W/m2, air velocity 1.5 m/sec,inlet temperature of 35oC and different valuesof mass flow rate.
Validation
Validation and verification focus on thecom-parison between the predicted output andthe experimental results for each type of solarair heater. The experimental results have beencarried out by Karim et al. (2004), for V-grooveabsorber in single and double pass mode. Theoutput from the mathematical simulation hasbeen run according to the same configuration andparameters used in experimental work done byKarim et al. (2004). A great correlation hasoccurred between the experimental and thepredicted efficiencies (R2 = 99.5%, P < 0.001)for single flow mode as shown in Figure 3. Alsoa good correlation (R2 = 98.7%, P < 0.001) and(R2 = 97.5%, P < 0.001) has been found betweenthe experimental and predicted efficiencies andoutlet temperature respectively as shown inFigure 4.
This study of double pass V-grooveabsorber with porous media has notbeen conducted before either theoretically orexperi-mentally. The theoretical study has beenconducted by using the program developedin this study.
Results and Discussion
Figure 5 shows the variation of efficiency withmass flow rate for V-groove absorber in singlepass, double pass and double pass mode withporous media. From the figures, it can be seenthat the efficiency of the air heater is stronglydependent on the air flow rate. The efficienciesof all three air heaters increased constantly, thentended to approach a constant value. This figureclearly shows that the double flow mode is4 - 7% more efficient than the single flow mode.This increase in efficiency in double pass modeis due to the increased heat removal from twoflow channels compared to one flow channel insingle pass operation. On the other hand, the useof porous media in double flow increases the airheater efficiency to 7% more efficiency than theair heater in single mode and more than 2 - 3%efficiency in double flow mode without porousmedia. Hence, the use of porous media increasesthe heat transfer area which contributed to
45Suranaree J. Sci. Technol. Vol. 14 No. 1; January - March 2007
Figure 2. Simulation procedure flow chart
Conceptualization Phase
Knowledge representationKnowledge acquisition
46 Performance Analysis for V-groove Absorber
the higher efficiency.Hence, the outlet temperature is an
important parameter for drying applications; theoutlet temperature was investigated for a widerange of flow rates. Figure 6 shows the variationof outlet temperature with flow rate. As expectedthe outlet temperature of the flowing air throughthe collector decreased with increased flow rate,but after a flow rate of about 0.05 kg/sec forsingle flow mode, and a flow of about 0.065kg/sec for double flow mode the rate of thetemperature drop became lower. Figure 7
illustrates the variation of the pressure dropwith mass flow rate; it shows that the pressuredrop is a function of the mass flow rate; hence itis increased by increasing the mass flow rate.The figure clearly shows that the value of thepressure drop in double flow mode almostdoubles the value of the pressure drop in singleflow. At the same time the use of porous mediain the double flow V-groove absorber increasesthe pressure drop from 3 to 25 Pa more than thepressure drop in the double flow V-grooveabsorber without the porous media.
Predicted efficiencyExperimental efficiency
Mass flow rate (kg/s)
Eff
icie
ncy
Predicted TemperatureExperimental TemperaturePredicted EfficiencyExperimental Efficiency
Mass flow rate (kg/s)
Eff
icie
ncy
oute
t te
mpe
ratu
re (
o C)
Figure 4. Experimental and predicted output for double flow mode
Figure 3. Experimental and predicted output for single flow mode
47Suranaree J. Sci. Technol. Vol. 14 No. 1; January - March 2007
Eff
icie
ncy
Mass flow rate (kg/s)
Single passDouble passDouble pass with porous media
Figure 5. Efficiency variation with mass flow rate for single pass, double pass and doublepass with porous media in V-groove absorber
Mass flow rate (kg/s)
Single passDouble passDouble pass with porous media
Out
let
tem
pera
ture
(o C
)
Figure 6. Outlet temperature variation with mass flow rate for single pass, double passand double pass with porous media in V-groove absorber
The variation in efficiency and pressuredrop with flow channel depth for singlepass mode in V-groove absorber at differentcollector lengths (1.8 m, 1.5 m, and 1 m.) isdisplayed in Figures 8 - 9 respectively at a fixedmass flow rate of 0.034 kg/sec. Figure 8indicates that at a fixed mass flow rate theefficiency decreases with the increase of the flowchannel depth. Science the deep air channel depthreduces the heat transfer to the flowing air whichresulting in low efficiency of the system, and thiseffect is more predominant for a longer flow
channel. Figure 9 illustrates that the pressuredrop increases with a decrease in the flow depth,and this increase is greater for a longer channelflow. The variation in outlet temperature with theflow channel depth is displayed in Figure 10,which indicates that the outlet temperatureincreases with the decrease in the flow depth.
The effect of a different upper channeldepth on the pressure drop, efficiency and theoutlet temperature for double pass mode with andwithout porous media is conducted, for a fixedmass flow rate of 0.03 kg/sec, a constant lower
48 Performance Analysis for V-groove Absorber
flow depth of 0.025 m channel and two differentchannel lengths of 1.0 m and 1.5 m. It is found thatwith the increase of the flow depth the pressuredrop decreases and also the efficiency and theoutlet temperature decreases and by increasingthe duct length, the efficiency decreases but theoutlet temperature and the pressure dropincreases as illustrated in Figures 11 - 13. Itappears from Figures 11 - 13 that the use of theporous media increases the system efficiencyby 3 - 7%, but the rise in outlet temperature isfaintly low at 2 - 3oC.
Conclusion
A mathematical simulation to predict the effectof different parameters on system thermalperformance and pressure drop, for V-grooveabsorbers in single and double flow modes withand without using a porous media have beenconducted. It is found that increasing the massflow rate through the air heaters results in higherefficiency but also pressure drop is increased.On the other hand, decreasing the channel flowdepth results in increasing the system efficiency
Mass flow rate (kg/s)
Pre
ssur
e dr
op (
Pa)
Single passDouble passDouble pass with porous media
Figure 7. Pressure drop variation with mass flow rate for single pass, double pass and doublepass with porous media in V-groove absorber
Flow depth (m)
Eff
icie
ncy
mm
m
Figure 8. Efficiency variation with flow channel depth for single pass in V-groove absorber
49Suranaree J. Sci. Technol. Vol. 14 No. 1; January - March 2007
Flow depth (m)
Out
let
tem
prat
uer
(o C)
Figure 10. Outlet temperature drop variation with flow channel depth for single pass inV-groove absorber
and outlet temperature at the same time as itincreases the pressure drop.
The channel length also has an effect onthe thermal efficiency, hence the efficiency ofthe system is increases more for a shortchannel length than for a long one, while thepressure drop is less than the pressure drop fora long channel length.
The double flow is more efficient thanthe single flow mode and the use of porousmedia increases the system efficiency and the
outlet temperature. This increment will result inan increase in the pressure drop thus increasingthe pumping power expanded in the collector.
Finally the adoption of the aforemen-tioned mathematical simulation that has beendeveloped to predict the thermal performanceof solar air heaters, and to find the influence ofdifferent parameters seems to be promising, as itis capable of predictive a reasonable resultsaccording to chosen parameters with design rulesthat incorporate human expertise in this field.
Figure 9. Pressure drop variation with flow channel depth for single pass in V-grooveabsorber
Flow depth (m)
Pre
ssur
e dr
op (
Pa)
mm
m
50 Performance Analysis for V-groove Absorber
Upper channel depth (m)
Eff
icie
ncy
With porous media L = 1mWith porous media L = 1.5mWithout porous media L = 1mWithout porous media L = 1.5m
Figure 12. The variation of efficiency with upper channel depth
Upper channel depth (m)
Without porous media L = 1mWithout porous media L = 1.5mWith porous media L = 1mWith porous media L = 1.5m
Pre
ssur
e dr
op (
Pa)
Figure 11. The variation of pressure drop with upper channel depth
Figure 13. The variation of outlet temperature with upper channel depth
Upper channel depth (m)
Out
let
tem
pera
ture
(o C
)
With porous media L = 1mWith porous media L = 1.5mWithout porous media L = 1mWithout porous media L = 1.5m
51Suranaree J. Sci. Technol. Vol. 14 No. 1; January - March 2007
Furthermore, the use of the internet will helpin sharing and distributing this knowledge.
This process should be of interestto designers and engineers who would like tocompare the cost effectiveness of solar airheaters.
Notations
A Area of collector that absorb solarradiation, m2
CP
Specific heat of working fluid, J/kg KD Collector Width, mDh Hydraulic diameter, mf Friction factorFR Heat removal factorh Fluid heat transfer coefficient, W/m2 Khr Radiation heat transfer coefficient,
W/m2 KH
VV-groove height, m
I Solar radiation, W/m2
L Collector length, mm Collector flow rate, kg/secN u Nusselt numberP Pressure drop across the duct, PaQ u Rate of useful energy gain, WRe Reynolds numberT a Ambient air temperature, KT c Cover temperature, KT f Fluid temperature, KT i Fluid inlet temperature, KT p Absorber plate surface temperature, KT pr Porous media temperature, KT r Bottom plate temperature, KU Overall heat loss coefficient, W/m2 KU b Back loss coefficient, W/m2 KU t Top loss coefficient, W/m2 KW Collector width, m
Greek symbols
ε p Emittance of absorber plateε c Emittance of glass coverρ Air density, kg/m3
σ Stephen-Boltzman constantτ Solar transmittance of glazingα Solar absorptance of collector plateφ V-groove angleµ Dynamic viscosity, Pa.sec
Subscripts
a ambientb backc coverf fluidh hydraulicp plateu useful
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