PREPARATION,CHARACTERISATION ENGINE PERFORMANCE AND EMISSION CHARACTERISTICS OF COCON
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PREPARATION,CHARACTERISATION ENGINE PERFORMANCE AND EMISSION CHARACTERISTICS OF COCONUT-OIL BASED HYBRID FUELS
College Of Engineering, Trivandrum
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The double-pass solar collector with porous media in the lower channel provides a higher outlet temperature compared to the conventional single-pass collector. Therefore, the thermal efficiency of the solar collector is higher. A theoretical model has been developed for the double-pass solar collector. An experimental setup has been designed and constructed. The porous media has been arranged in different porosities to increase heat transfer, area density and the total heat transfer rate. Comparisons of the theoretical and the experimental results have been conducted. Such comparisons include the outlet temperatures and thermal efficiencies of the solar collector for various design and operating conditions. The relationships include the effect of changes in upper and lower channel depth on the thermal efficiency with and without porous media. Moreover, the effects of mass flow rate, solar radiation, and temperature rises on the thermal efficiency of the double-pass solar collector have been studied. In addition, heat transfer and pressure drop relationships have been developed for airflow through the porous media. Close agreement has been obtained between the theoretical and experimental results. The study concluded that the presence of porous media in the second channel increases the outlet temperature, therefore increases the thermal efficiency of the systems.
List of figures/graphs 2
1. Introduction 3
1.1) Problem Definition
1.1.1) Solar Collector
1.1.2) Porous Medium
1.2.2) Experimental Study
2. Objectives 8
3. Mathematical Model 9
3.3) Equations and Analysis
3.4) Analysis Data
4. Experimental Setup 17
5. Experimental Procedures 18
6. Error Analysis 18
7. Results and Observations 19
7.1) Variations of Experimental and Theoretical Efficiencies
With Different Porosities
7.2) Effect Of Solar Radiation on Thermal Efficiency
7.3) Effect Of Temperature Rise on Thermal Efficiency
7.4) Effect Of Solar Radiation on Temperature and Thermal Efficiency
7.5) Comparison between Experimental and Theoretical Nusselt Number
7.6) Effects of Reynolds’s Number on Friction Factor
8. Conclusions 26
9. References 27
Symbol Description Unit
A Area of Solar Air Heater m^2
B Width of the Collector m
C Specific Heat J/kg K
d_1 First Channel Depth m
d_2 Second Channel Depth m
G Rate of Airflow kg/s
Gr Grashof Number -
h Heat Transfer Coefficient W/m^2 K
I Solar Radiation W/m^2
k Thermal Conductivity W/m K
L Length of the Collector m
M Mass per unit Area kg/m^2
m Number of Harmonics -
Nu Nusselt Number -
Pr Prandtl number -
Re Reynolds number -
T Temperature K
t Time s
U_R Real Loss Factor W/m^2 K
V Velocity m/s
x Coordinate -
α_g Glass Absorptivity -
δ Thickness -
τ Glass Transmittance -
σ Stefan’s Boltzmann constant W/m^2 K^4
α_p Plate Absorptivity -
ε Emissivity -
η Efficiency -
μ Kinematic viscosity of air kg/m s
ϕ Porosity -
1&2 Referred to First and Second Stream
b Black plate
pm Porous media
LIST OF FIGURES/GRAPHS
Index Page No
Fig 1- Schematic Model of Double Pass Solar Collector 10
Fig 2- Schematic of the Experimental Setup 17
Fig 3- Variations of the experimental and theoretical efficiencies 19
on the double-pass solar collector with porous media(∅=70%)
Fig 4- Variations of the experimental and theoretical efficiencies 20
on the double-pass solar collector with porous media(∅=90%)
Fig 5- Effect of Solar Radiation on the Thermal Efficiency on the Double 21
Pass Solar Collector with Porous Media (∅=80%,Ta=33.5℃)
Fig 6- Effect of Solar Radiation on the Thermal Efficiency on the Double 22
Pass Solar Collector with Porous Media (∅=90%,Ta=33.5℃)
Fig 7- Effect of Solar Radiation on Temperature Rise and Thermal Efficiency 23
Fig 8- Comparison between the experimental and theoretical Nusselt number 24
on the double pass solar collector with porous media
Fig 9- Effect of the Reynolds’s Number on the Friction Factor on the Double 26
Pass Solar Collector with Porous Media
1.1) Problem Definition
1.1.1) Solar Collector
A solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers or simpler installations such as solar air heat. The more complex collectors are generally used in solar power plants where solar heat is used to generate electricity by heating water to produce steam which drives a turbine connected to an electrical generator. The simpler collectors are typically used for supplemental space heating in residential and commercial buildings. A collector is a device for converting the energy in solar radiation into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The solar energy striking the Earth's surface depends on weather conditions, as well as location and orientation of the surface, but overall, it averages about 1,000 watts per square meter under clear skies with the surface directly perpendicular to the sun's rays.
1.1.2) Porous Medium
A porous medium (or a porous material) is a material containing pores (voids). The skeletal portion of the material is often called the "matrix" or "frame". The pores are typically filled with a fluid (liquid or gas). The skeletal material is usually a solid, but structures like foams are often also usefully analyzed using concept of porous media.
A porous medium is most often characterized by its porosity. Other properties of the medium (e.g., permeability, tensile strength, electrical conductivity) can sometimes be derived from the respective properties of its constituents (solid matrix and fluid) and the media porosity and pores structure, but such a derivation is usually complex. Even the concept of porosity is only straight-forward for a poroelastic medium.
Often both the solid matrix and the pore network (also known as the pore space) are continuous, so as to form two interpenetrating continua such as in a sponge. However, there is also a concept of closed porosity and effective porosity, i.e., the pore space accessible to flow.
Many natural substances such as rocks, soils, biological tissues (e.g. bones, wood), and manmade materials such as cements and ceramics can be considered as porous media. Many of their important properties can only be rationalized by considering them to be porous media.
Several configurations of solar air heaters (SAHs) have been developed in literature in searching of a suitable design for different types of applications, various designs of solar collectors have been the subject of many theoretical and experimental investigations. There are many alternative designs to the conventional single-pass collector. These designs must be able to reduce the heat
losses from the collector to increase in the operating temperature and collector efficiencies of the system. Therefore, single-pass solar collector with porous media has been introduced. Inexpensive porous materials such as stones, crushed glass, wool, and metal wool have been used for application in developing countries. The double-flow types of SAHs have been introduced for increasing the heat transfer area, leading to improve thermal performance.
This increases the thermal energy between the absorber plate and the air, which clearly improves the thermal performances of the solar collectors with obstacles arranged into the air channel
duct. These obstacles allow a good distribution of the fluid flow.
1.2.2) Experimental Study
A simulation study of a double-pass solar collector with porous media has been analyzed by Mohamad. The main idea is to minimize heat losses from the front cover of the collector and to maximize heat extraction from the absorber. Forcing air to flow over the front glass cover (preheat the air) before passing through the absorber can achieve this objective. Hence, this design needs an extra cover to form a counter-flow heat exchanger. Porous media forms an extensive area for heat transfer, where the volumetric heat transfer coefficient is very high; it will enhance heat transfer from the absorber to the airstream. In the design of this type of collector, which combines double air passage and porous media, pressure drop should be minimized. However, the thermal efficiency of this type of collector is significantly higher than the thermal efficiency of conventional air heaters, exceeding 75% under normal operating conditions. The pressure drop is not so significant if high porous medium is used and careful design of U-return
section is considered.
Sopian et al. conducted experimental studies on the double pass solar collector with and without porous media in the second channel. The collector has only one glass cover and a blackened metal absorber and the material used as the porous media is steel wool. To ensure good flow distribution across the bed it is necessary that the pressure drop across the packed bed is large. However, it is also necessary to keep the pressure drop low so that the energy spent in pumping the air through the bed is low to make the system cost effective. The use of a double-pass resulted in increasing the pressure drop across the collector. However, the increase in the operating cost due to the increased pressure drop in the collector is considered small. This is due to the fact that the pressure drop across the collector is small compared to the total pressure drop across the system.
Mohamad, who referred the conventional double-pass collector as a counter-current solar collector, showed that the thermal efficiency can be improved by 18% compared to the conventional solar heater. The study also suggested an extra fan power of 2–3W, which is not high. Also noted that the cost of construction of the double glazing collector is comparable to the cost of the double-pass collector. However, necessary trade-off between the fan power and the efficiency of packed bed solar collector must be analyzed to obtain a cost-effective design configuration. The thermal performance of a double-glass double-pass solar air heater with a packed bed (DPSAHPB) above the heater absorber plate was investigated experimentally and theoretically by Ramadan et al. Limestone and gravel were used as packed bed materials. Numerical calculations were carried out, on typical summer days of 2003, to study the effect of different operational and configurational parameters on the heater performance. Effects of the mass flow rate of air and the mass and porosity of the packed bed material were also studied. It was inferred that for increasing the outlet temperature Tfo of the flowing air after sunset, it is advisable to use the packed bed materials with higher masses and therefore with low porosities. It is recommended to operate the system with packed bed with values of equal 0.05 kg/s or lower to have a lower pressure drop across the system.
To evaluate the thermal efficiency ,by energy balance and fundamental heat transfer equations, of a double pass solar collector and bring out the heat transfer and the pressure drop relationships for air flow through porous medium.
To compare the properties like outlet temp and thermal efficiency of a formulated model with experimental results for various designs and operating conditions
To study the changes in thermal efficiency for variations in the upper and lower channel depths , with and without porous medium
To throw light on the effects of mass flow rate, solar radiation and temperature rises on the thermal efficiency of the collector, with and without porous medium
3. MATHEMATICAL MODEL
To simplify the analysis, the energy balance equations are written under the following assumptions:
The temperatures of the cover and plates vary only in the direction of fluid
The side losses are negligible and leakage of air to/or from the collectors is negligible
Ideal gas with constant specific heat.
An unsteady-state mathematical model has been developed. This involves unsteady-state energy balance equations linking the outer glass cover heat transfer coefficient, and the heat transfer coefficients between the moving airstreams and surfaces forming the upper and lower channels.
Fig 1. (a) Schematic of the double-pass solar collector with porous media
Fig 1. (b) Schematic of heat transfer coefficients in the double-pass solar collector.
3.3) Equations and Analysis
Heat balance through glass cover can be described by
Airflow between the glass cover and the plate can be written as
Heat balance through the absorber plate can be written as
Eq. (3) assumes that the air and solid matrix are in thermal equilibrium, i.e. the temperature of the solid is equal to the temperature of the air locally. The value of the effective thermal conductivity (k_pm) is a function of porosity, the thermal conductivity of the solid material and the thermal conductivity of air and its value is of the order of 10 times the thermal conductivity of fluid. The value of thermal conductivity is set to 300 W/m K which is about 10 times the thermal conductivity of the air.
The airflow in the lower channel can be described by
Heat balance at the bottom is written as
The boundary conditions are obtained from the conditions that there is no heat loss from the side of the metallic plates. One of the boundary conditions state that at entry point, air temperature equals ambient temperature such as
Eqns. (1) – (5) are subjected to the following boundary conditions:
Radiative and convective heat transfer coefficients should be known in order to solve above equations. Radiative heat transfer coefficient is a function of the surfaces temperature of both sides, while convective heat transfer coefficient is a function of the dimensionless parameter known as Nusselt number (Nu).
where D_h is the equivalent (hydraulic) diameter of the duct. In the case of noncircular cross-sections without porous media, D_h is given by
In the case with porous media, it can be written as
Nu which is the Nusselt number is a function of Reynolds number of the flow, which is given by
The flow can be divided into three regimes as
Laminar flow regime (Re< 2300):
where the constants are a =0.00190, b =0.00563, m=1.71, n =1.17, Pr=0.7, and 〖Nu〗_∞ =5.4
(b) Transition flow regime (2300 <Re< 6000):
where μ is evaluated at film temperature and μ_w is evaluated at wall temperature.
© Turbulent flow regime (Re> 6000):
where Prandtl number Pr is given as
The determination of the average heat transfer coefficient, h, between the porous media and air as follow:
Pressure drop or lost head is directly proportional to the length of the duct, proportional to the square of the flow rate, and proportional to the fifth power of the duct size. Therefore, the ductwork designer can be relatively unconcerned about the length of the run, only moderately concerned with the circulation rate, but must be extremely sensitive that the size of the duct is appropriate for the flow rate. Consequently, the pressure drop through the collectors is of highest interest since that is where the minimum dimensions are most likely to be found. We now discuss the characteristics of friction factor in fluid in duct runs in the collector. An important fundamental relationship is the Fanning equation, given here in a modified form as
where ∆P is the frictional loss or pressure drop, G_f is the fluid mass flow rate, L is the length of the duct, 〖A 〗_x is cross-sectional area, g_c is a constant (1 kg m/N s^2 or 32.17 ft/s^2), ρ is the fluid density, and f is a friction factor. The friction factor, f, is as empirical function of the relative roughness of the duct.
For smooth duct, when the flow is laminar the friction factor is given by
And when it is turbulent is given by
The thermal performance of solar air heater can be expressed as
3.4) Analysis Data
The following values of physical parameters have been used:
B = 1.2 m, L = 2.2 m, 〖 d〗_1 = 0.07 m, d_2 = 0.07 m, 〖 m〗_g = 5.5 kg/m^2,
m_p = 6.55 kg/m^2, C_f= 1012 J/kg K, Cg =840 J/kg K, Cp= 500 J/kg K,
k_g = 0.0263 W/m K, k_p = 237 W/m K, k_b= 116 W/m K, 〖 α〗_g= 0.06,
α_p= 0.95, δ_g = 0.004 m, ε_g = 0.92, and τ_g =0.92.
4. EXPERIMENTAL SETUP
Fig 2. Schematic of the experimental setup with the solar simulator
Fig. 2 shows the solar simulator and the collector undergoing testing. The simulator uses 45 halogen lamps, each with a rated power of 300W. The maximum average radiation of 642 W/m_2 can be reached. Dimmers are used to control the amount of radiation that the test collector received. The dimmers are divided into six scales for producing different amount of radiation values. These values have been previously measured using the pyranometer. The measurement errors are about 3.16% for radiation value of 277.8 W/ m2 and 4.05% for radiation value of 642 W/m_2. A heater is placed at the inlet of the collected undergoing test to vary the inlet temperature.
The collector consists of the glass cover, the insulated container and the black painted aluminum absorber. The size of the collector is 120 cm in width and 240 cm in length. The first and second channels can be adjusted for optimal operations. The inlet temperature to the collector can be adjusted by heating the inlet air to the collector. Thermocouples are located strategically at the inlet, end-of-the first pass, outlet, absorber plate and glass cover. The temperature measurements are recorded using data acquisition system. The flow rates are measured using the vane type anemometer.
5. EXPERIMENTAL PROCEDURES
The lighting control of the simulator is adjusted to obtain the required radiation levels. The solar collector is operated at varying inlet temperature, airflow rate, channel depth, and radiation conditions. Air is circulated for 30 min prior to the period in which data are taken. The depths for the upper (d_1) and lower (d_2) channels are varied. The upper channel is varied from 3.5 cm to 10.5 cm and the lower channel is varied from 7 cm to 14 cm. The mass flow rate, G, is varied from 0.03 kg/s to 0.07 kg/s. The porosity of the porous media has been changed for each set of experiments.
6. ERROR ANALYSIS
The error during the experiment as follows: the mass flow rate is about 3.2%, temperature rise is about 2.8%, area of the collector according to the measurements is about 1.8%, and the pressure drop through the collector was measure by a micro-manometer has an error of about 0.5%. Therefore, the error on calculating the thermal efficiency of the system is about 8%.
7. RESULTS AND OBSERVATIONS
7.1) Comparison of Experimental and Theoretical Efficiency on Double Pass Solar Collector for Different Porosities
Fig 3. Variations of the experimental and theoretical efficiencies on the double-pass
solar collector with porous media (∅ = 70%, Ta = 33.5 °C)
Fig 4. Variations of the experimental and theoretical efficiencies on the double-pass
solar collector with porous media (∅ = 90%, Ta = 33.5 °C)
Figs. 3 and 4 give a comparison between the theoretical and experimental efficiency for the double-pass solar collector with saturated porous media, which varies from 70% to 90% under solar radiation from 546 W/m^2 to 614 W/m^2. The thermal efficiency is quite close between the theoretical and experimental values for solar radiation less than 570 W/m^2, but the deviation becomes more pronounced for solar radiation greater than or equal to 590 W/m^2. Also, it can be seen that the thermal efficiency increases with an increase in the solar radiation.
7.2) Effect of the Solar Radiation on the Thermal Efficiency on the Double Pass Solar Collector with Porous Media
Fig 5. Effect of the solar radiation on the thermal efficiency on the double-pass solar
collector with porous media (∅ = 80%, Ta = 33.5 °C).
Fig. 5 shows the effect of mass flow rate on the thermal efficiency and temperature rise when the porosity is 80%. The collector thermal efficiency increases with an increase in the mass flow rate, while the temperature rise decreases with the corresponding increase in the mass flow rate. The collector thermal efficiency increases as a result of a decrease in the average temperature of the absorber plate. The latter arises because of the increase in the flow rate cause by an increase in the heat transfer coefficient between the channel walls and the air. As is evidence in the figure, the efficiency of the solar collector with porous media is about 10% better than without porous media. This is because of the increase in the area of the heat transfer analogous to the increase in heat transfer by using extended surfaces such as fins and corrugated absorber plate.
7.3) Effect of Temperature Rise on the Thermal Efficiency on the Double Pass Solar Collector with Porous Media
Fig 6. Effect of temperature rise on the thermal efficiency on the double-pass solar
collector with porous media (Ta = 33.5 °C).
Fig. 6 shows the effect of temperature rise on the thermal efficiency of double-pass solar collector with media of 90% porosity. The thermal efficiency increases with the increase in the temperature rise.
7.4) Effect of Solar Radiation on Temperature Rise and Thermal Efficiencies
Fig 7. The effect of solar radiation on temperature rise and thermal efficiencies
Fig. 7 shows the effect of solar incident on the temperature rise and thermal efficiency. The collector thermal efficiency increases with an increase in the solar radiation, while the temperature rise decreases with increasing solar radiation. Also, it can seen that the solar collector with porous media has a higher thermal efficiency than the collector without porous media for an increase in solar radiation. Therefore, introducing porous media in the second channel increases the heat transfer area correspondingly increasing the heat transfer coefficient.
7.5) Comparison between the Experimental and Theoretical Nusselt Number on the Double Pass Solar Collector with Porous Media
Fig 8. Comparison between the experimental and theoretical Nusselt number on the
double pass solar collector with porous media
Fig. 8 shows the theoretical and experimental results of Reynolds number and Nusselt number of the double-pass solar collector with porous media. Using porous media and for increasing solar radiation the effect on the performance of the solar collector is improved. This is because increasing the heat transfer area leads to an increase in the heat transfer coefficient. Also the same effect is seen for a decrease in the thermal conductivity.
7.6) Effect of the Reynold’s Number on the Friction Factor of the Double Pass Solar Collector with Porous Media
Fig 9. Effect of the Reynolds’s number on the friction factor on the
double pass solar collector with porous media
Fig. 9 shows the effect of the Reynolds number on the friction factor. Porous media in the lower channel can be used to increase the heat transfer coefficient since the friction factor depends on the losses and velocity of the airflow rate.
Porous medium increases the area for heat transfer and increases the heat transfer coefficient between the plate and fluid.
Addition of porous medium in the second channel increases the efficiency by 10%
The experimental results had a close agreement with the theoretical analysis
Typical thermal efficiency of the double pass solar collector with porous media is 60-70%
Medium with a porosity of 90% gave maximum efficiency.
Upper channel depth of 3.5cm and Lower channel depth of 10.5cm gave maximum efficiency with a mass flow rate of 0.07kg/s
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4) Ramadan MRI, El-Sebaii AA, Aboul-Enein S, El-Bialy E. Thermal performance of a packed bed double-pass solar air heater. Energy 2007;8:1524–35
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