Chapter 1 Flashcards
How does the science of heat transfer differ from the science of thermodynamics?
The science of thermodynamics deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to another and makes no reference to how long the process will take. But in engineering, we are often interested in the rate of heat transfer, which is the topic of the science of heat transfer.
What is the caloric theory? When and why was it abandoned?
The caloric theory asserts that heat is a fluid- like substance called the caloric that is a massless, colourless, odourless, and tasteless substance that can be poured from one body into another. When caloric was added to a body, its temperature increased; and when caloric was removed from a body, its temperature decreased. It maintained that heat is a substance that could not be created or destroyed. Yet it was known that heat can be generated indefinitely by rubbing one’s hands together or rubbing two pieces of wood together. 19th century
How do rating problems in heat transfer differ from sizing problems?
The rating problems deal with the determination of the heat transfer rate for an existing system at a specified temperature difference. The sizing problems deal with the determination of the size of a system in order to transfer heat at a specified rate for a specified temperature difference.
What is the difference between the analytical and experimental approach to heat transfer? Discuss the advantages and disadvantages of each approach
The experimental approach (testing and taking measurements) has the advantage of
dealing with the actual physical system and getting a physical value within the limits of
experimental error. However, this approach is expensive, time-consuming, and often
impractical. The analytical approach (analysis or calculations) has the advantage that it is
fast and inexpensive, but the results obtained are subject to the accuracy of the
assumptions and idealizations made in the analysis.
What is the importance of modelling in engineering? How are the mathematical models for engineering processes prepared?
Physical problem = > identify variables, make assumptions and approximations, apply relevant physical laws
When modelling an engineering process, how is the right choice made between a simple but crude and a complex but accurate model? Is the complex model necessarily a better choice since it is more accurate?
The right choice depends on the situation at hand. The right choice is usually the simplest model that yields adequate results. For example, the process of baking potatoes or roasting a round chunk of beef in an oven can be studied analytically in a simple way by modelling the potato or the roast as a spherical solid ball that has the properties of water (Fig. 1–5). The model is quite simple, but the results obtained are sufficiently accurate for most practical purposes. As another example, when we analyze the heat losses from a building in order to select the right size for a heater, we determine the heat losses under anticipated worst conditions and select a furnace that will provide sufficient heat to make up for those losses.
What is heat flux? How is it related to the heat transfer rate?
The rate of heat transfer per unit area normal to the direction of heat transfer is called heat flux, and the average heat flux is expressed as q = Q / A
What are the mechanisms of energy transfer to a closed system? How is heat transfer distinguished from the other forms of energy transfer?
Energy can be transferred to or from a given mass by two mechanisms: heat Q and work W. An energy interaction is heat transfer if its driving force is a temperature difference. Otherwise, it is work.
How are heat, internal energy, and thermal energy related to each other?
In daily life, we frequently refer to the sensible and latent forms of internal energy as heat, and we talk about the heat content of bodies. In thermodynamics, however, those forms of energy are usually referred to as thermal energy to prevent any confusion with heat transfer.
An ideal gas is heated from 50°C to 80°C (a) at constant volume and (b) at constant pressure. For which case do you think the energy required will be greater? Why?
For an ideal gas at constant pressure, it takes more heat to achieve the same increase in temperature than it does at constant volume. The specific heat at constant pressure Cp is greater than Cv because at constant pressure the system is allowed to expand and the energy for this expansion work must also be supplied to the system. For ideal gases, these two specific heats are related to each other by Cp =Cv+R.
What is the physical mechanism of heat conduction in a solid, a liquid, and a gas?
In solids, conduction is due to the combination of the vibrations of the molecules in a
lattice and the energy transport by free electrons. In gases and liquids, it is due to the
collisions of the molecules during their random motion.
Write down the expressions for the physical laws that govern each mode of heat transfer, and identify the variables involved in each relation.
Conduction:
Q=-kA (dT/dx) k=thermal conductivity
A= Area dT= T° difference dx = thickness
Convection:
Q= hAs(Ts-Tinf) h= conv heat transfer coef
As = Area SURFACE Ts=T° surface Tinf= T° far
Radiation:
Q= εσAsT^4 ε=emissivity σ=constant
As= Area SURFACE Ts= T°surface
How does heat conduction differ from convection?
Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion. The faster the fluid motion, the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction.
Does any of the energy of the sun reach the earth by
conduction or convection?
No only by radiation
How does forced convection differ from natural convection?
Convection is called forced convection if the fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. In contrast, convection is called natural (or free) convection if the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid.
