Chapter 1 Terms & Concepts

What is the difference between heat flux and a heat rate? What are their units?

Heat Flux is the rate of heat transfer per unit area and its units are W/m^2 or Btu/(ft^2 - hr). Heat rate is thermal energy transfer per unit time and its units are W or Btu/hr.

What is the inherent difference between the application of conservation of energy over a time and at an instant of time?

In a time interval we consider the equation until the whole of the body is at steady state i.e. the temp between the bodies has come to equilibrium. In an instant of time we consider finite element analysis when there is a small increase in temp i.e dT.

What is the difference between natural convection and forced convection?

In natural convection, the flow is induced by the differences between fluid densities which result due to temperature changes. Forced convection uses externally induced flow, such as wind.

What is irradiation? What are its units?

Irradiation is the process by which an object is exposed to ionizing radiation. Gray (Gy) is the unit of ionizing radiation and is defined as absorption of 1 joule of radiation energy per kg of matter.

What is Newton's law of cooling?

Newton's law of cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its surroundings provided the temperature difference is small and the nature of radiating surface remains same.

Surface of a solid at an elevated temperature and exposed to cooler surroundings. By what mode(s) is heat transferred from the surface if (1) it is in intimate (perfect) contact with another solid, (2) if 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?

1) Conduction because conduction occurs due to temperature difference in a solid or in a surface (not motion). 2) Convection because in convection heat flows due to motion (flow of liquid). 3) Convection (same reason as above, heat transfer due to flow of gas). 4) Radiation (radiation is a process in which energetic particles or energetic waves travel through a vacuum).

What is the thermal conductivity? What are its units? What role does it play in heat transfer?

A measure of the ability of a material to transfer heat. Given two surfaces on either side of a material with a temperature difference between them, the thermal conductivity is the heat energy transferred per unit time and per unit surface area, divided by the temperature difference.

What two outcomes characterize the response of an opaque surface to incident radiation? What outcome affects the thermal energy of the medium bounded by the surface and how? What property characterizes their outcome?

Absorption and Reflection

What are the physical mechanisms associated with heat transfer by conduction, convection, and radiation?

Conduction is the transfer of heat through a solid material. Convection is the transfer of heat by moving air. Thermal radiation is the transfer of heat in the form of electromagnetic waves.

What effect does convection heat transfer from or to a surface have on the solid bounded by the surface?

Convection is equal to the conduction in the surface.

What is predicted by the Stefan-Boltzmann law, and what unit of temperature must be used with the law?

E= sigma *(Ts)^4 absolute temp must be used (K)

What is the emissivity, and what role does it play in characterizing radiation transfer at a surface?

Emissivity is the ability of a body to emit all the radiation falling on it. It is very important in radiation as it determines the rate of heat transfer. The greater the emmisivity the greater the heat transfer.

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?

Spatial Temperature Difference

What is a temperature gradient? What are its units? What is the relationship of heat flow to a temperature gradient?

Temperature gradient is in Fourier's Law as the change in temperature as a function of distance and its units are dT/dx.

What conditions are associated with use of the radiation heat transfer coefficient?

The body must have a high emissivity. The temperature of the body must be bulk temperature i.e. whole body is at same temperature.

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?

The hydrodynamic boundary layer is a region of a fluid flow, near a solid surface, where the flow patterns are directly influenced by viscous drag from the surface wall. The Thermal Boundary Layer is a region of a fluid flow, near a solid surface, where the fluid temperatures are directly influenced by heating or cooling from the surface wall.A thermal boundary layer occurs in the theory of heat transfer. Temperature differs across a thermal boundary layer. The two boundary layers may be expected to have similar characteristics but do not normally coincide. Liquid metals tend to conduct heat from the wall easily and temperature changes are observed well outside the dynamic boundary layer. Other materials tend to show velocity changes well outside the thermal layer. A thermal boundary layer that determines the thermodynamic interaction of heat transfer is necessary for the development of a hydrodynamic boundary layer.

What is Fourier's law?

The law of heat conduction, also known as Fourier's law, states that the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat flows.

What is thermal energy storage? How does it differ from thermal energy generation? What role do the terms play in a surface energy balance?

Thermal energy storage comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. They can be employed to balance energy demand between day time and night time. However a common thermal energy generation process involves the conversion from electrical to thermal energy in a current carrying medium (resistance heating).

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?

Under steady condition, temperature varies with location in the medium. It remains fixed with respect to time.

Write the equation used to express net radiation exchange between a small isothermal surface and a large isothermal enclosure.

heat exchange = sigma x emmisivity x (T2^4 - T1^4) where sigma = boltzman constant

What role is played by the convection heat transfer coefficient in Newton's law of cooling? What are its units?

q = h(Ta - Tb) units: W/m^2-K