ASEN 3713 Exam 3 T/F

Surfaces that are intended to remain cool under the Sun, such as the surface of a radiator on a spacecraft, are desired to have high solar absorptivity and low emissivity in the infrared region.

False, you want these things to cool, so you want low solar absorptivity and high emissivity (so it doesn't absorb heat and then rejects heat easily)

An incandescent light bulb operating at 2800 K emits about a quarter of its radiation energy in visible light.

False, you need much higher heat to get get that much visible light (like the sun)

The spectral irradiation is defined as the rate at which radiation of wavelength l is incident on a surface per unit area of the surface and per unit wavelength interval dl about l. For an opaque medium, the spectral irradiation is partially absorbed and partially transmitted

False, the first part is true, but opaque means no transmission.

Unlike emissivity, the total absorptivity of a material is practically independent of surface temperature but depends strongly on the temperature of the source at which the incident radiation is originating.

True, absorptivity is what the material takes in so it depends on the source whereas emissivity depends on the material itself

The reflection from surfaces can be idealized as diffuse (uniformly in all directions, rough surfaces) or specular (mirror like, polished surfaces).

True, we idealize these to make calculations easier. Diffuse is a rough surface, so when light hits it it bounces all around while a specular surface reflects light in one direction.

The view factor Fij is defined as the fraction of the radiation leaving surface i that is intercepted by surface j. Thus, the view factor from a surface to itself will be zero, regardless of the shape of the surface.

False, Fij is the fraction from i hitting j, but Fii or Fjj can be nonzero, in concave surfaces.

The network representation of the diffuse-gray surface serves as a useful tool for visualizing and calculating radiation exchange.

True, it turns it into a basic circuit problem

CO2 and water vapor in the atmosphere transmit the bulk of solar radiation but absorbs the infrared radiation emitted by the surface of the Earth.

True, greenhouse effect. H2O and CO2 are mostly transparent to shortwave radiation but absorbs longwave like infrared.

Convection heat transfer from a solid surface to a fluid is merely the conduction heat transfer from the solid surface to the fluid layer adjacent to the surface (when the no-slip condition is met). Heat is then convected away from the surface as a result of fluid motion.

True, at the surface is conduction and away from the surface (in the fluid) is convection. No movement would mean pure conduction.

A Nusselt number of Nu=1 for a fluid layer represents heat transfer across the layer by pure conduction (excluding radiation here)

True, a nusselt number is how much total heat transfer is there compared to pure conduction. If Nu>1 convection is also present while with Nu = 1, conduction HT = total HT.

The Nusselt number is defined as Nu= hLc/k, where Lc is the characteristic length. The Nusselt number provides a measure of the convection coefficient. The larger the Nusselt number, the more effective is the conduction.

False, that is the right equation for nusselt number, but the larger the number the LESS effective is the conduction. Nu = convection / conduction.

For heat transfer between a surface and an adjacent fluid when they are at different temperature, the dominant contribution is generally made by the bulk or fluid motion of the fluid particles, although molecular motion contributes to this transfer as well.

True, molecular motion is always present, but bulk fluid motion is most influential most of the time as molecule to molecule conduction has very slow diffusion. (ex: conduction is passing water bucket to bucket while convection is putting it on a conveyor belt.)

The hydrodynamic boundary layer is the thin region next to the surface in which the velocity of the fluid, V, changes from zero at the surface (no slip condition) to the free stream velocity, V∞, some distance from the surface. The boundary layer thickness is defined as the distance from the surface to that distance at which V=0.5V∞.

False, they part about the hydrodynamic boundary layer is correct, but V = 0.5Vinf is not always right. it is normally defined as V = 0.99 V inf

The heat transfer between a surface and an adjacent fluid with a different temperature is greater in the laminar flow region than in the turbulent region

False, it is greater in the turbulent region as there is more chaotic motion that better causes the heat to 'mix in'. laminar is very smooth and relies much more on slow conduction which transfers less heat.

For forced convection the Nusselt number is a function of the Reynolds number and the Prandtl number.

True, Re captures how fluid moves while Pr captures how heat diffuses vs momentum diffuses, including the componants you need to solve Nu (which represents how much convection improves HT over conduction). AKA Re+Pr successfully describe fluid motion and how heat behaves within the moving fluid.