# Daily Archives: December 5, 2020

## Determine the total heat transfer and the outlet temperatures of water and oil. (cp of water = 4186 J/kgK).

A.    Water enters a counterflow double-pipe heat exchanger at 15ºC flowing at the rate of 1700 kg/h. It is heated by oil (cp = 2000 J/kgK) flowing at the rate of 550 kg/h from an inlet temperature of 94°C. For an area of 1 m2 and an overall heat transfer coefficient of 1075 W/m2K, determine the total heat transfer and the outlet temperatures of water and oil. (cp of water = 4186 J/kgK).

B.    A simple counterflow heat exchanger operates under the following conditions: Fluid A: inlet and outlet temperatures 80°C and 40°C, Fluid B: inlet and outlet temperatures 20°C and 40°C. The exchanger is cleaned, causing an increase in the overall heat transfer coefficient by 10% and inlet temperature of fluid B is changed to….

## Derive an equation relating the temperatures of oil and water at any section of the heater.

A.    A tubular heater of the counterflow type is used to heat 1.26 kg/s of fuel oil (cp = 3.14 kJ/kg K) from 10∞C to 26.7∞C. Heat is supplied by means of 1.51 kg/s of water which enters the heater at 82∞C.

(a) Derive an equation relating the temperatures of oil and water at any section of the heater.

(b) Determine the necessary surface area if the overall heat transfer coefficient is 1.135 kW/m2 K.

B.    A one shell pass, two tube pass heat exchanger has a total surface area of 5 m2, and its overall heat transfer coefficient based on that area is found to be 1400 W/ m2 K. If 4500 kg/h of water enters the shell side at 315∞C while 9000 kg/h of….

## Determine the necessary surface area if the overall heat transfer coefficient is 1.135 kW/m2 K.

A.    A tubular heater of the counterflow type is used to heat 1.26 kg/s of fuel oil (cp = 3.14 kJ/kg K) from 10∞C to 26.7∞C. Heat is supplied by means of 1.51 kg/s of water which enters the heater at 82∞C.

(a)   Derive an equation relating the temperatures of oil and water at any section of the heater.

(b)   Determine the necessary surface area if the overall heat transfer coefficient is 1.135 kW/m2 K.

B.    Water is evaporated continuously at 100°C in an evaporator by cooling 500 kg of air per hour from 260∞C to 150∞C. Calculate the heat transfer surface area required and the steam evaporation per hour, if the liquid enters at 100∞C. Take U0 = 46 W/m2 K and cp of air….

## Explain transpiration cooling. What is film cooling?

A.      Explain transpiration cooling. What is film cooling?

B.      What do you understand by ablative cooling? What are its applications?

C.      What is ablative velocity? What is heat of ablation?

D.      A thermocouple 0.063 cm in diameter is held normal to an air stream, and at M = 0.7, it reads 60°C. If the recovery factor is 0.9, find the static temperature of the air. 9.4 Air at 1/20 atm and 275 K flows with a free stream velocity of 700 m/s over a flat plate 1.2 m long. If the surface of the plate is to be maintained at 325 K, determine the amount of cooling needed per metre width of the plate.

## At what distance x is the temperature equal to 60°C?

A.      A scale model of an aeroplane wing section is tested in a wind tunnel at M = 1.5. The air pressure and temperature in the test section are 20 kPa and – 30°C, respectively. If the wing section is to be kept at an average temperature of 60°C, determine the rate of cooling required. The wing model can be approximated by a fl at plate of 0.3 m length in the fl ow direction.

B.      For a heat flux (Q/A)0 = 4 ¥ 106 W/m2 and the following material properties, estimate the steady-state ablation velocity and the fraction Qc/Q0 in 30s. At what distance x is the temperature equal to 60°C? In 30 show much material has ablated? Given: r = 1602 kg/m3, c = 1256….

## How is mass transfer coefficient evaluated by dimensional analysis?

A.     How is mass transfer coefficient evaluated by dimensional analysis?

B.     What is Sherwood number? What is its counterpart in heat transfer?

C.     Bring out the analogy of heat transfer and mass transfer.

D.    When direct mass transfer data for a system are not available, show how the mass transfer coefficient can be predicted from the relevant heat transfer data.

E.     For boundary-layer flow over a fl at plate, how is the mass transfer coefficient related to heat transfer coefficient and skin friction coefficient?

## Take the mass diffusivity for N2 – CO2 mixture as 0.16 ¥ 10–4 m2/s.

Two large vessels contain uniform mixtures of nitrogen (component A) and carbon dioxide (component B) at 1 atm, T = 289 K, but at different concentrations. Vessel 1 contains 90% N2 and 10% CO2 by moles, whereas vessel 2 contains 20% N2 and 80% CO2 by moles. The two vessels are connected by a duct of 0.1524 m ID, and 1.22 m long. Determine the rate of transfer of nitrogen between the two vessels by assuming that steady-state transfer takes place in view of the large capacity of the two reservoirs. Take the mass diffusivity for N2 – CO2 mixture as 0.16 ¥ 10–4 m2/s.

## Calculate the time required for 1 kg of N2 to diffuse across 1 sq. m. of this air film.

A.    One method of measuring diffusion coefficients of vapours is to measure the rate of evaporation of a liquid in narrow tubes. In one such experiment, a glass tube 1 cm in diameter was filled with water at 20°C to within 4 cm of the top. Dry air at 20°C and 1.013 bar was blown across the top of the tube. At the end of 24 hours of steady-state operation, the level of the water dropped 0.1 cm. Calculate the diffusivity of the air-vapour system at 20°C.

B.    Nitrogen diffuses steadily through a stagnant layer of air which is 0.5 cm thick. The concentration of N2 is 0.08 kg/m3 at one face and zero at the other. The total pressure is 1 atm and the temperature 20°C…..

## Estimate the amount of moisture evaporated per hour, assuming that the block of ice is perfectly insulated except for the surface exposed to the air stream.

A.    Predict the mass transfer coefficient for liquid ammonia vaporising into air at 1 atm, 25°C, knowing that the heat transfer coefficient in the same equipment, at the same gas and

liquid flow rates, is 4.544 kW/m2 K.

B.    Dry air at 25°C and atmospheric pressure blows over a 30 cm2 surface of ice at a velocity of 1.5 m/s. Estimate the amount of moisture evaporated per hour, assuming that the block of ice is perfectly insulated except for the surface exposed to the air stream.

## Determine the mass transfer coefficient for the transfer of naphthalene from the pipe surface into the air in regions away from the inlet.

A.    Air at 10°C and 1 atm flows over a plane surface covered with naphthalene. The flow velocity is such that the Reynolds number at a distance of 0.6 m from the leading edge of the plate is 9 ¥ 104. Determine the average mass transfer coefficient for the transfer of napthelene over the 0.6 m length of the surface by making use of the analogy between heat and mass transfer in laminar flow along a flat plate.

B.    Air at 25°C and atmospheric pressure flows with a velocity of 7.6 m/s inside a 2.5 cm inner diameter pipe. The inside sur face of the tube contains a deposit of naphthalene. Determine the mass transfer coefficient for the transfer of naphthalene from the pipe surface into the….