## Summer2022-V M335 Homework 9

热传导作业代写 Determine corresponding values of the total, hemi-spherical emissivity ε and the total emissive power E at 2000 K.

### 12.37

Consider the metallic surface of Example 12.7. Addi-tional measurements of the spectral, hemispherical emissivity yield a spectral distribution which may be approximated as follows:

(a) Determine corresponding values of the total, hemi-spherical emissivity ε and the total emissive power E at 2000 K.

### 12.62 热传导作业代写

The spectral transmissivity of plain and tinted glass can be approximated as follows:

Plain glass:T_{λ}=0.9 0.3≤λ≤2.5 μm

Tinted glass: T_{λ}=0.9 0.5≤λ≤1.5 μm

Outside the specified wavelength ranges, the spectral transmissivity is zero for both glasses. Compare the solar energy that could be transmitted through the glasses.With solar irradiation on the glasses, compare the visi-ble radiant energy that could be transmitted.

12.64 The spectral absorptivity α_{λ} and spectral reflectivity P_{λ} for a spectrally selective, diffuse material are as shown.

(a) Sketch the spectral transmissivity τ_{λ}.

(b) If solar irradiation with Gs = 750 W/m^{2 }and the spectral distribution of a blackbody at 5800K is incident on this material, determine the fractions of the irradiation that are transmitted, reflected,and absorbed by the material. 热传导作业代写

(c) If the temperature of this material is 350 K, deter-mine the emissivity ε.

(d) Determine the net heat fux by radiation to the material.

### 12.69 热传导作业代写

An opaque，horizontal plate has a thickness of L= 21 mm and thermal conductivity k = 25 W/m.K.Water flows adjacent to the bottom of the plate and is at a temperature of T_{∞，w}= 25°C. Air flows above the plate at T_{∞，α}= 260°C with h_{a}= 40 W/m^{2}.K.The top of the plate is diffuse and is irradiated with G = 1450 W/m^{2}， of which 435 W/m^{2} is reflected.The steady-state top and bottom plate temperatures are T_{t}= 43°C and T_{b}= 35°C, respectively. Determine the transmissivity, reflectivity, absorptivity, and emis-sivity of the plate. Is the plate gray? What is the radiosity associated with the top of the plate? What is the convection heat transfer coefficient associated with the water flow?

12.71 A diffuse surface having the following spectral charac-teristics is maintained at 500 K when situated in a large furnace enclosure whose walls are maintained at 1500 K:

(a) Sketch the spectral distribution of the surface emissive power E_{λ} and the emissive power E_{λ,b }that the surface would have if it were a blackbody.

(b) Neglecting convection effects, what is the net heat flux to the surface for the prescribed conditions?

### 12.95 热传导作业代写

A procedure for measuring the thermal conductivity of solids at elevated temperatures involves placement of a sample at the bottom of a large furnace. The sample is of thickness L and is placed in a square container of width W on a side. The sides are well insulated. The walls of the cavity are maintained at T_{w}, while the bot-tom surface of the sample is maintained at a much lower temperature T_{c }by circulating coolant through the sample container. The sample surface is diffuse and gray with an emissivity_{ε}_{s.}Its temperature T_{s}is measured optically. 热传导作业代写

(a) Neglecting convection effects, obtain an expres-sion from which the sample thermal conductivity may be evaluated in terms of measured and known quantities (T_{w},T_{s}, Te, εs, L). The measure-ments are made under steady-state conditions.If T_{w} = 1400K，T_{s}= 1000K，εs= 0.85，L =0.015m, and T_{c}= 300K，what is the sample thermal conductivity?

12.114 A contractor must select a roof covering material from the two diffuse, opaque coatings with α_{λ}(λ) as shown. Which of the two coatings would result in a lower roof temperature? Which is preferred for sum-mer use? For winter use? Sketch the spectral distribu-tion of ax that would be ideal for summer use. For winter use.

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