Heat Transfer

Provide descriptive (~ 0.5 - 1 page) answer(s) to each of the following questions (use Figures and provide sample calculation for each law wherever feasible to illustrate your response). Your answers should not be too mathematical/complex but at a level that an engineering under- graduate student can understand.

Question # 3 a) Discuss main challenges and limitations in the design and operation of heat exchangers. b) How the performance of heat exchanger can be enhanced.

Question 5 In a polymeric pipe (50 W/m/K of thermal conductivity) 13 m long, the temperature measured at the outermost radius is 11°C. The temperature at the innermost pipe surface is 57°C and the heat flux at outermost surface is one quarter of the heat flux at innermost surface of the pipe. Evaluate the whole heat transfer (absolute value) through the pipe thickness to the nearest kW. Note: use pi=3.1415

Question 6 A phase change fluid at 100°C is pumped through 3 m of a multi-layered pipe, which is exposed to an external fluid current at 22°C. The innermost layer is made of steel (thermal conductivity 13 W/m/K), with inner and outer radii 40 mm and 52 mm, respectively. The second layer is made of insulating material (thermal conductivity 0.6 W/m/K), with outer radius equal to 70 mm. The convective heat transfer coefficients associated to the internal and external fluids are respectively 1,329 W/m2/K and 150 W/m2/K. Calculate the heat transfer (absolute value) through the pipe thickness in kW to 1 decimal place. Note: use pi-3.1415

Question 7 An electrical circuit is embedded in a silicone slab, which has a thickness equal to 0.06 m. If the heat flux dissipated towards the surrounding amounts 3.2 kW/m2 on each slab side, evaluate the volumetric heat generation in kW/m3 to 1 decimal unit. Suggestion: the temperature field across the system is symmetric.

Choose the correct statements from the options below. There may be several correct statements, but you won't know how many. If there are N correct statement, you will get 5/N marks for each correct statement you choose, but you will lose 5/N marks for each incorrect statement you choose. Your score won't go negative.

1. (2 pts) Short-answer and Multiple-choice Problems A. When Bi << 1, which of the following is approximately true for the temperature, 7, of the object being transiently cooled? Explain your reasoning. a. T = f(t) only b. T = f(t) and one spatial dimension c. T = f(t, x,y,z) B. Consider a plate of material X, originally at To, that is exposed to fluid at T<To and a given heat-transfer coefficient h at time t = 0. If the plate is now replaced with material Y with a higher thermal conductivity but the same plate size and given heat-transfer coefficient, how will the time for the center of the plate to cool to a given value change with the new material Y? C. Cite two reasons that insulation is used for chemical engineering process equipment. D. Consider a hot oven with an insulated door exposed to a cold room. If the room temperature was increased but all other variables were not changed, would (i) the heat loss to the room increase, decrease, or not change? (ii) the temperature of the outside surface of the door increase, decrease or not change? Explain your reasoning.

A hot surface at 78°C exposed to air at 16°C is to be cooled by attaching 16-cm-long and 1.7-cm-diamotor cylindrical fins. The convection heat transfer coefficient is 29 Wim2 K, and heat transfer from the fin tip is negligible. If the fin efficiency is 0.71, calculate the rate of heat loss from 71 fins, in Wi

1. (1 pt) Multiple-choice Questions A. In a system consisting of species A and B, for which condition are N₁ and J₁ equal or nearly equal? a When N= N b. When Ng=-N₁ c. When N * 0, and species A is very dilute (x<<1) d. When (b) or (c) is true B. For flow of a cool fluid through a tube with hot walls, would decreasing the flow rate of the fluid though the tube cause the heat-transfer rate to a. increase? b. decrease? c. not change?

PROBLEM 1 Problem 10.5. A circular heater plate with 80 mm diameter is placed in a tank containing liquid nitrogen at 1 MPa pressure and 80 K temperature. The upward facing side of the plate is maintained at 100 K. Find the heat transfer rate between the heater and liquid nitrogen. For liquid nitrogen properties, you may assume: p = 745.6 kg/m³, Cp = 2.122; ‚μ = 104 x 10-6N.2, k = 0.122- kJ kg.K m². 0.0072 K-¹. W m.K , Pr = 1.80,ß =