ASEN 3113 Exam 2 - Clickers & Quiz Questions

T/F - An alloy made of two metals of thermal conductivities k1 and k2 usually has a thermal conductivity k between k1 and k2.

False

T/F - Bodies at a temperature below 0 oC do not emit thermal radiation.

False

T/F - Considering steady one-dimensional heat flow through a plane wall of thickness L and thermal conductivity k that is exposed to convection on both sides to fluids at temperature T∞1 and T∞2 with heat transfer coefficients h1 and h2, respectively, we need to know the surface temperatures of the wall in order to evaluate the rate of heat transfer through it.

False

T/F - For a (cylindrical) wire with higher temperature than the surrounding to be insulated with an insulating material that has a thermal conductivity k, the thicker is the insulation layer, the smaller is the heat loss.

False

T/F - In SI units, thermal conductivity, k, has the unit of [W/(m2·K)].

False

T/F - Like other ideal cycles discussed before, the ideal vapor-compression refrigeration cycle is also regarded as an internally reversible cycle.

False

T/F - Silicon oils and gaskets are commonly used in the packaging of electronic components because they provide both good thermal contact and good electrical conduction.

False

T/F - The back work ratio for a gas turbine is much smaller than that for a steam power plant.

False

T/F - The cold-air standard assumptions include the air-standard assumptions while employing constant specific heats whose values are determined at freezing temperatures.

False

T/F - The saturation temperature of water steam at 20 kPa is about 60oC. It is not possible to maintain a pressure of 20 kPa in a condenser that is being cooled by river water entering at 20oC.

False

T/F - The thermal contact resistance is greater for smooth surfaces than rough surfaces.

False

T/F - The thermal efficiency of a diesel engine is greater than that of an Otto engine for the same compression and specific heat ratio.

False

T/F - The thermal resistance for conduction through a plane wall is proportional to the area of the wall and inversely proportional to thickness of the wall.

False

T/F - When a temperature difference exists between two surfaces, a net heat transfer cannot occur in the absence of any intervening medium.

False

T/F - With respect to an air-standard analysis, one of the assumptions is that the combustion process is replaced by a heat-rejection process that restores the working fluid to its initial state.

False

T/F - The thermal conductivity of Aluminum is almost twice as large as the thermal conductivity of Copper at room temperature.

False (it's the opposite)

T/F - For the house electric wire, the plastic insulation layer functions as insulating the electric current and also to reduce the heat loss.

False (we want to increase heat transfer, do not want heat loss)

T/F - Thermodynamic cycles always involve a phase transition during the cycle.

False - not always

After you touched both a wood bar and an iron/steel bar placed outside in the winter overnight with your bare hand, which one would have a higher temperature?

Iron/steel bar (has a higher heat conductivity so more heat is transferred from our body/hand to the iron/steel)

After a cold-blooded snake slithered over both a wood bar and an iron/steel bar placed outside in the winter overnight, which bar would have a higher temperature?

The bars would be the same temperature (snake's body temperature is equivalent to that of both bars, so there is no heat transfer)

T/F - A material that has a high thermal conductivity or a low heat capacity will have a large thermal diffusivity. The larger the thermal conductivity, the faster the propagation of heat into the medium.

True

T/F - A wood bar and iron/steel bar placed outside near by in the winter overnight (lets say about 5C). They have the same temperature.

True

T/F - All four components associated with the Rankine cycle (the pump, boiler, turbine, and condenser) are steady-flow devices, and thus all four processes that make up the Rankine cycle can be analyzed as steady-flow processes.

True

T/F - Any excessive moisture at the final stages of the turbine should be avoided in a vapor power plant (a Rankine cycle), one way to reduce the moisture level is to expand the steam in the turbine in two stages, and reheat it in between.

True

T/F - Finned surfaces are commonly used in practice to enhance heat transfer.

True

T/F - Forced convection inside the International Space Station is required.

True

T/F - Generally speaking, for both ideal and actual cycles, thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system or with a decrease in the average temperature at which heat is rejected from the system.

True

T/F - Heat transfer is energy in transit due to a temperature difference.

True

T/F - In gas-turbine engines, the temperature of the exhaust gas leaving the turbine is often considerably higher than the temperature of the air leaving the compressor.

True

T/F - It is a common experience to feel 'chilly' in winter and 'warmer' in summer in our homes even when the thermostat is kept the same. This is due to the so called 'radiation effect' resulting from radiation heat exchange between our bodies and the surrounding surfaces of the wall and the ceiling.

True

T/F - Kirchhoff's law of radiation states that the emissivity and the absorptivity of a surface at a given temperature and wavelength are equal.

True

T/F - On a T-s diagram, a heat-addition process proceeds in the direction of increasing entropy, a heat-rejection process proceeds in the direction of deceasing entropy.

True

T/F - The average fluid temperature should be as low as possible during heat rejection in order to increase the thermal efficiency of a steam power plant. The condensers of steam power plants usually operate well below the atmospheric pressure.

True

T/F - The back work ratio for a gas turbine power plant is defined as the ratio of the compressor work to the turbine work.

True

T/F - The compression process in an ideal cycle (e.g., Otto, Diesel, and Brayton) is internally reversible and adiabatic, and thus isentropic. The actual compression process, however, involves frictional effects, which increase the entropy, and heat transfer, which may increase of decrease the entropy, depending on the direction.

True

T/F - The displacement volume is the volume displaced by the piston as it moves between TDC and BDC.

True

T/F - The ideal Rankine cycle does not involve any internal irreversibilities and consists of the following four processes: 1-2 Isentropic compression in a pump, 2-3 Constant pressure heat addition in a boiler, 3-4 Isentropic expansion in a turbine, 4-1 Constant pressure heat rejection in a condenser.

True

T/F - The thermal contact resistance at the two 1-cm-thick aluminum plates can be greater than the sum of the thermal resistances of both plates.

True

T/F - The thermal efficiency of a Carnot cycle is independent of the type of working fluid used (ideal gas, steam, etc.) or whether the cycle is executed in a closed or steady-flow system.

True

T/F - The thermal resistance concept is widely used in practice because it is intuitively easy to understand and it has proven to be a useful tool in the solution of a wide range of heat transfer problems. But its use is limited to systems through which the rate of heat transfer remains constant.

True

T/F - There are different types of heat transfer processes. We refer to them as modes: conduction, convection, and radiation.

True

T/F - In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction.

True and False. True if you're only considering convection and conduction, False because you can have radiation if there is a temperature difference.