Consider the cylindrical receiver in a solar thermal power plant shown below. The receiver is made of opaque material and has a diameter D = 8m and length L = 14m. At a particular time, the heliostats direct a concentrated solar flux of q”S = 80 kW/m2 onto the receiver. The ambient air has a temperature T( = 300K and there is no wind.

Under these conditions, the surface of the receiver has a temperature of Ts = 700K, an emissivity of ( = 0.3 and a solar absorptivity (S = 0.8.

i) Calculate the heat loss from the vertical surface of the receiver due to convection. Assume effects of curvature are negligible.

ii) Calculate the heat loss from the vertical surface of the receiver due to radiative emission. Neglect irradiation from the surroundings.

iii) Determine the collector efficiency.

iv) If a wind started to blow, what would happen to the surface temperature of the receiver? Would you expect the collector efficiency to increase, decrease or remain the same? Explain your answers. (Note – assume all other conditions remain the same.)

[pic]

Question 2

The roof of a refrigerated truck compartment is of composite construction, consisting of a layer of foamed urethane insulation (t2 = 40mm, ki = 0.03W/mK) sandwiched between aluminium alloy panels (t1 = 6mm, kp = 200W/mK). The length and width of the roof are L = 12m and W = 4m respectively. The temperature of the refrigerated space within the truck is –15ºC. The solar absorptivity and the emissivity of the outer surface are (S = 0.3 and ( = 0.7.

Consider conditions for which the truck is moving at a speed of V = 90km/h, the air temperature is T( = 30ºC and the solar irradiation is GS = 900W/m2. Assume turbulent flow over the...

...DEFINITION OF HEATTRANSFER |
Heattransfer is energy in transit due to temperature difference . Whenever there exists a temperature difference in a medium or between media, heattransfer must occur. The basic requirement for heattransfer is the presence of temperature difference . There can be no net heattransfer between two mediums that are at the same temperature. The temperature difference is the driving force for heattransfer, just as the voltage difference is the driving force for electric current flow and pressure difference is the driving force for fluid flow. The rate of heattransfer in a certain direction depends on the magnitude of the temperature gradient (the temperature difference per unit length or the rate of change of temperature) in that direction. The larger the temperature gradient, the higher the rate of heattransfer |
PHYSICAL ORIGINS AND RATE EQUATIONS:It is important to understand the physical mechanisms which underlie the heattransfer modes and that we are able to use the rate equations that quantify the amount of energy being transferred per unit time. Conduction:Conduction can be imagined as a atomic or molecular activity which involves the transfer of energy from the more...

...Introduction to Heat and Temperature
Gonzalo Leon
Strayer University
2012/01/27
Introduction to Heat and Temperature
Relation between the Study of Heat and Kinetic Theory
Kinetic theory can be describes as a scientific theory of the movement of an object. Kinetic theory relates to capacity of a subject to do work on another object due to their motion. Kinetic theory of matter explains that the same is compose of tiny pieces of, atoms or molecules in continues motion. The theory states that the actions of matter inside an object and the actions heat generates. Kinetic theory explains as well the temperature transition by the means of transmission, where thermal powers shows to be conducted throughout matter, heating up cooler regions. This reaction is the delivery of heat or heat flow. Kinetic theory relates to the study of heat due to the fact that once heat is produced, kinetic energy is created as well. Heat energy can convert or transfer its self into kinetic energy that could make a subject move.
Definition of Heat and Temperature
Heat:
Heat could simply be summarize as been energy. It is the delivery of Kinetic energy from one subject to a different one. Heat is conveyed in three different ways: radiation, convection and conduction. An example of...

...Biology Coursework Practical Heat Loss
The aim of this practical is to find to what extent does the surface area to volume ratio of an object affect the rate of heat loss from the object
Hypothesis: As the ratio of surface area to volume of an object decreases the rate of heat loss from the object will also decrease. Objects with the same surface area to volume ratios loose heat at the same rate so long as there are not other variables involved.
Background Knowledge:
The surface area to volume ratio of an object is determined by dividing the surface area by the volume and putting it into a ratio of one.
e.g. A flask of volume 200cm3 and a surface area of 160cm2 will have a surface area to volume ratio of:
160
---- : 1 = 1.25 : 1
200
Objects that are not the same size but have the same surface area to volume ratios loose heat at the same rate. So a flask, with a volume of 200cm3 with a surface area of 160cm2 and a surface area to volume ratio of 1.25:1, will loose heat at the same rate as a similar flask of volume 625 and a surface area of 500 which also has a surface area to volume ratio of 1.25:1. However, generally when you increase the size of an object the surface area to volume ratio decreases so in this example it is very likely that the two flasks in question are different shapes.
In this experiment the two flasks which will be used will...

...Mechanisms of HeatTransfer
Prepared by: Ms. Ana Antoniette C. Illahi
1
Conduction
• conduction (or heat conduction) is the transfer of thermal energy between regions of matter due to a temperature gradient. Heat spontaneously flows from a region of higher temperature to a region of lower temperature, and reduces temperature differences over time, approaching thermal equilibrium.
Prepared by: Ms. Ana Antoniette C. Illahi
2
(Heat Current in Conduction)
• • • • • • • • H - Heat Current dQ – Quantity of Heat dt – Time dQ/dt – the rate of heat flows A – Cross sectional area (TH - TC) – Temperature difference L – Length k – constant (thermal conductivity)
Prepared by: Ms. Ana Antoniette C. Illahi
H = dQ/dt = kA (TH - TC)/L
3
Conduction
H = dQ/dt = -kA (dT/ dx) H = A(TH - TC) / R R = L/ k R – thermal resistance
Prepared by: Ms. Ana Antoniette C. Illahi 4
Thermal Conductivities k (W/m oC)
Metals Aluminum Brass Copper Lead Mercury Silver Steel 205.0 109.0 385.0 34.7 8.3 406.0 50.2
Prepared by: Ms. Ana Antoniette C. Illahi 5
Solids (representative values) Brick. Insulating 0.15 Brick. red 0.6
Concrete 0.8
Cork Felt Fiberglass Glass Ice Rock wool Styrofoam Wood
0.04 0.04 0.04 0.8 1.6 0.04 0.01 0.12-0.04
Prepared by: Ms. Ana Antoniette C. Illahi 6
Gases Air Argon Helium Hydrogen Oxygen
0.024 0.016...

...HEATTRANSFERHeattransfer, also known as heat flow, heat exchange, or simply heat, is the transfer of thermal energy from one region of matter or a physical system to another. When an object is at a different temperature from its surroundings, heattransfer occurs so that the body and the surroundings reach the same temperature at thermal equilibrium. Such spontaneous heattransfer always occurs from a region of high temperature to another region of lower temperature, as required by the second law of thermodynamics.
In engineering, energy transfer by heat between objects is classified as occurring by heat conduction, also called diffusion, of two objects in contact; fluid convection, which is the mixing of hot and cold fluid regions; or thermal radiation, the transmission of electromagnetic radiation described by black body theory. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heattransfer.
II. THREE MODES OF HEATTRANSFER
1. Conduction
In heattransfer, conduction (or heat conduction) is the transfer of thermal energy between neighboring molecules in a substance...

...FREESTUDY
HEATTRANSFER
TUTORIAL 2
CONVECTION AND RADIATION
This is the second tutorial in the series on basic heattransfer theory plus some elements of advanced
theory. The tutorials are designed to bring the student to a level where he or she can solve problems
ranging from basic level to dealing with practical heat exchangers.
On completion of this tutorial the student should be able to do the following.
•
•
Explain the use of the surface heattransfer coefficient.
•
Explain the use of the overall heattransfer coefficient.
•
Combine convection and conduction theory to solve problems involving
flat, cylindrical and spherical surfaces.
•
Explain the basic theory behind radiated heattransfer.
•
Explain the affect of the emissivity and shape of the surface.
•
Calculate effective surface heattransfer coefficient.
•
(c) D. J. Dunn
Explain natural and forced convection.
Solve basic problems involving convection and radiation.
1
CONVECTION
Convection is the study of heattransfer between a fluid and a solid body. Natural convection occurs
when there is no forced flow of the fluid. Forced convection occurs when the fluid is forced to flow
over the object.
Consider a hot vertical surface placed in a...

...Unsteady State HeatTransfer laboratory were to study the rates of heattransfer for different materials of varying sizes, to develop an understanding of the concepts of forced and free convection and to determine the heattransfer coefficients for several rods. These objectives were met by heating several rods and allowing them to cool through free convection in air, free convection in water and forced convection in water- while monitoring their change in temperature over change in time.
Seven heattransfer coefficients were determined during the laboratory for various rods. A copper rod underwent free convection in air, free convection in water and forced convection in water. The measured heattransfer coefficients for the copper trials were 10.13 W/m2K, 438.43 W/m2K and 1715.69 W/m2K, respectively. These results supported theory that convection occurs for quickly in denser mediums and when it has a driving force. Two stainless steel rods underwent forced convection in water; the large rod had an experimental heattransfer coefficient of 1704.42 W/m2K while the small one had 1817.43 W/m2K. The smaller rod was expected to have the larger heattransfer coefficient since it has a smaller surface area. The results of the stainless steel rods therefore also supported theory....

...HEATTRANSFER
LECTURE 1
OUTLINE
Introduction
Heat Conduction
Forced Convection
Natural Convection
Application of Conduction & Convection
Thermal Radiation
Application of Radiation
Introduction
What does the subject of heattransfer
deal with? And why it is important.
What does the subject of mass transfer
deal with?
Heattransfer is the science that deals with
the study of rates of exchange of heat
between hot and cold bodies.
The hot and cold bodies are called source
and receiver respectively.
In all such cases temperature difference
between a source and the receiver is the
driving force for heattransfer.
Difference between TD and HT
The former deals with heat and other forms of energy.
Whereas, the latter is concerned with the analysis of the rate
of heattransfer.
TD deals with systems in equilibrium so it cannot be used to
predict how fast a change will take place since a system is
not in equilibrium during the process, but it may predict the
amount of energy required to change the system form one
equilibrium state to another.
Ex:- cooling of hot steel bar place in a jar of water
Why it is important to study HT?
Once we have the LAWS which govern...