A Fill in the blanks (16 points) I. In steady-state, a flat-wall consists f thre

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A Fill in the blanks (16 points) I. In steady-state, a flat-wall consists f three layers. Given: thickness Bi>B>B3, thermal conductivity kickisk. Try to Given: thic conduction q w, 2. In a flat wall constructed of a series of layers, in steady heat fo the larger the temperature drop in this layer is. 3. In film-type the heat res the heat resistance in a layer is condensation, the bigger the temperature drop between two sides of condensing film is, the coefficient of condensing heat transfer is. 4.In order to improve the heat-sinking) capability of electric instrument, you can spray oil painting with on the surface of it. 5. In a shell-and-tube exchanger, a liquid was cooled by frozen brine with-10C. The fuid should be aranged in side B ? to 50C. For countercurrent flow, ATm 2 Some liquid is flowing through a ?25x2.5 mm at a turbulent flow. Its coefficient of convection heat transfer is Multiple choices (24 points) a double-pipe exchanger, hot flow was cooled from 90C to 60C and the temperature of cool flow was from 20 ?)70 ?) 30 C) 10 C D) 40 C h-1000W/m2 C. If the velocity and physical properties are unchanged , the tube is changed to ? 192 mm, what is the h? A) 1059: B) 1496 C) 1585: D)1678 In a horizontal exchanger, stream is used to heat air. The air is in tube-side and the steam is i improve the overall heat transfer coefficient, which method is the best one? A. Improving the air flow C, Changing the flow direction of air and steam D, Putting the exchanger vertically 4. The emissivity of red brick is 0.93. The emissive power of it should be n shell-side. In order to B. Increasing the flow rate of steam B) 5686 W/m C) 916.8 WImim at 300 c D) 1000 W/m A)6112.3 W/m 5. The thermal conductivity of carbon steel isThe thermal conductivity of stainless steel isThe conductivity of copper is The thermal conductivity of air at atmospheric temperature is conductivity of water at atmospheric temperature is A)151 W/m. B)370 W/m. C) 45 W/m. D) 16 W/m.? E) 062 w/mC F) 0.025 wm.c 6. Given: The emissive power of refractory(???) is bigger than that of polished copper at T temperature. Then: The emissivity of refractory should be The thermal B) 0.03 C) 1 A) 0.85 7. The aim of dimensional analysis lies in A) Simplify the experiment and relevance work using the dimensionless groups instead of variables B) Get the exact quantitative relationship between dimensionless groups C) Get the exact quantitative relationship between variables D) Make the results more reliable using the dimensionless groups instead of variables 8. In steady-state, a flat-wall consists of three layers, as shown in figure. Try to compare the heat resistance Ri and (R2+R) A)R>(R2+R) B)R (R2+R3) C)R- (R2+R) D) Unable to compare

Explanation / Answer

Ans 1

From the Fourier's law of heat conduction

q = driving force / heat resistance

= change in temperature /R.......... Eq1

q = dT/ (B/k*A)

Heat resistance R = B/k*A........ from Eq 2

B = thickness

k = thermal conductivity

A = area of heat flow

Given that

B1 > B2 > B3

And

R1 < R2 < R3

From Eq 2

If B decreases ( from B1 to B3) and k increases (from k1 to k3) then R decreases.

R1 > R2 > R3

From Eq 1

If R decreases then q increases

q1 < q2 < q3

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