What are the physical mechanisms associated with heat transfer by conduction, co
ID: 2996371 • Letter: W
Question
What are the physical mechanisms associated with heat transfer by conduction, convection. and radiation? What is the driving potential for heat transfer? What are analogs to this potential and to heat transfer itself for the transport of electric charge? What is the difference between a heat flux and a heat rate? What are their units? What is a temperature gradient? What are its units? What is the relationship of heat flow to a temperature gradient? What is the thermal conductivity? What are its units? What role does it play in heat transfer? What is Fourier 's law? Can you write the equation from memory? If heat transfer by conduction through a medium occurs under steady-state conditions, will the temperature at a particular instant vary with location in the medium? Will the temperature at a particular location vary with time? What is the difference between natural convection and forced convection? What conditions are necessary for the development of a hydrodynamic boundary layer? A thermal boundary layer! What varies across a hydrodynamic boundary layer? Across a thermal boundary layer? If convection heat transfer for flow of a liquid or a vapor is not characterized by iquid/vapor phase change, what is the nature of the energy being transferred? What is it if there is such a phase change? What is Newton s law of cooling? Can you write the equation from memory? What role is played by the convection heat transfer coefficient in Newtons law of cooling? What are its units? What effect does convection heat transfer from or to a surface have on the solid bounded by the surface? What is predicted by the Stefan-Boltzmann law. and what unit of temperature must be used with the law? Can you write the equation from memory? What is the emissivity. and what role does it play in characterizing radiation transfer at a surface? What is irradiation? What are its units? What two outcomes characterize the response of an opaque surface to incident radiation? Which outcome affects the thermal energy of the medium bounded by the surface and how? What property characterizes this outcome? What conditions are associated with use of the radiation heat transfer coefficient? Can you write the equation used to express net radiation exchange between a small isothermal surface and a large isothermal enclosure? Consider the surface of a solid that is at an elevated temperature and exposed to cooler surroundings. By what mode(s) is heat transferred from the surface if (1) it is in inti¬mate (perfect) contact with another solid. (2) it is exposed to the flow of a liquid. (3) it is exposed to the flow of a gas. and (4) it is in an evacuated chamber? What is the inherent difference between the application of conservation of energy over a time interval and at an instant of time? What is thermal energy storage? How does it differ from thermal energy generation? What role do the terms play in a surface energy balance?Explanation / Answer
1) Mechanisms
Conduction or diffusion
The transfer of energy between objects that are in physical contact. Thermal conductivity is the property of a material to conduct heat and evaluated primarily in terms ofFourier's Law for heat conduction.
Convection
The transfer of energy between an object and its environment, due to fluid motion. The average temperature, is a reference for evaluating properties related to convective heat transfer.
Radiation
The transfer of energy from the movement of charged particles within atoms is converted to electromagnetic radiation.
2)
Heat transfer - the molecules (or atoms at a smaller level) of a substance which are on average moving faster are hotter (more heat). When these molecules come in contact with another substance with slower molecules they impact each other (either physically or through forces) and transfer some of their speed (kinetic energy) to the slower molecules. Thus the temperature decreases in the first substance and increases in the second substance.
Electric current - An Electro Motive Force (EMF) created by some means (chemically, magnetically, etc.) exerts a force on charged particles (electrons, protons, ions, etc.). This force causes movement of the charged particles when there is a complete loop back to the other side of the EMF device (such as a battery). Millions of these charged particles passing a point in the conductor per a unit time is measured as an ampere (amp). However, charged particle flow is really proportional to the overall flow of energy that is for all intent purposes instantaneous through the conductor (wire). For example, electrons flow (drift) relatively slow through a wire (in the range of meters per second, can't recall exact number)...however the energy flow is speed of light (or faster depending upon whom you believe. In conclusion all you can really say at this time is that current flow is a predefined number of charge carriers passing a point per unit time (direction can be either way or even both ways depending on the circuit).
Fluid flow - Fluid conforms to the shape of its container. So if we have a pipe it will "fill" all areas of the pipe. Fluid is made up a smaller parts (molecules of water for example). In a confined container if these molecules are moving faster (say you heat up the container) they are colliding with their container with more energy (average velocity) and frequency. This is measured as pressure. If you connect a water hose to this container the water will flow as long as the pressure of the water is higher then your outside air pressure. In effect the fluid will flow from the higher pressure to the lower pressure. A city water system increases the pressure by elevating the water container into the air and letting gravity pull the water down (the top of the tank is vented to atmospheric pressure to equalize). This would be an example of potential energy in a gravity field (and not pressure from heat (kinetic) as in the first example).
3)
Heat flux is the heat rate per unit area. If they gave you time, then they would ask for heat (J) not heat rate (J/s). Essentially what you are doing is dividing the equation for heat by time so that you dont need to know it to solve the problem. The whole point of a "rate" of anything per unit time is that it's independent of time. Speed is distance per time, but you can have a speed without knowing how long you go at that speed.
4)
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees (on a particular temperature scale) per unit length.fall in temperature in a body per unit distance is temperature gradient
5)
thermal conductivity (often denoted k, ?, or ?) is the property of a material to conduct heat. It is evaluated primarily in terms of Fourier's Law for heat conduction.
Heat transfer occurs at a higher rate across materials of high thermal conductivity than across materials of low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation. Thermal conductivity of materials is temperature dependent
6)
n a one dimensional differential form, Fourier's Law is as follows:
q = Q/A = -kdT/dx.
The symbol q is the heat flux, which is the heat per unit area, and it is a vector. Q is the heat rate. dT/dx is the thermal gradient in the direction of the flow. The minus sign is to show that the flow of heat is from hotter to colder.
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