Chapter 5, Problem 5.17P

A tube bank consists of 300 tubes at a distance of  6 cm between the centerlines of any two adjacent tubes. Air approaches the tube bank in the normal direction at 20°C and 1 atm with a mean velocity of  6.3 m/s. There are 20 rows in the flow direction with 15 tubes in each row with an average surface temperature of 140°C. For an outer tube diameter of 2 cm, determine the average heat transfer coefficient. Assume the air properties at mean temperature of 70°C and 1 atm. The air properties at the assumed mean temperature of 70°C and 1 atm are k = 0.02881 W/m·K, ρ = 1.028 kg/m3, cp = 1.007 kJ/kg·K, Pr = 0.7177, μ = 2.052 × 10−5 kg/m·s, Prs = Pr@Ts = 140°C = 0.7041.   The average heat transfer coefficient of the tube bank is  W/m2·°C.

In a dairy operation, milk at a flow rate of 0.25m3/hr and a cow-body temperature of 38.6°C must be chilled to a safe-to- store temperature of 13°C. Cold water at 2.2°C is available at a flow rate of 0.94m3/hr. The density and specific heat of milk are 1030 kg/m3 and 3860 J/kg.K, respectively. The density and specific heat of water is 1000 kg/m3 and 4187 J/kg.K. The chilling process is done by a double pipe counter-flow exchanger with an overall heat transfer coefficient U=1000 W/m2.K. The pipe of the heat exchanger has a 50-mm diameter with negligible thickness. Determine the pipe length L required. Select one: O a. 2.28 m O b. 4.28 m O c. 3.28 m O d. 1.28 m

Metal plates of size 60 cm × 50 cm, and thickness 1 cm are initially at 100◦C. They are to be cooled by blowing chilled air at 2 m/sec over both surfaces, the flow being directed along the side whose length is 60 cm. Incoming air temperature is 10◦C. Data for the metal: ρ = 7854 kg/m3, cp = 434 J/kg/K, k = 60.5 W/m/K Data for air: Use values at 50◦C. (a Determine the convective heat transfer coefficient at the surfaces of the plates.(b Determine whether lumped system analysis is applicable(c Assume the convection coefficient is constant, determine how much time is necessary for the temperature of the plates to drop to 25◦C.(d Estimate the total heat lost by a plate when it has reached 25◦C.

Using EES (or other) software, investigate the effect of the exhaust gas inlet temperature on the rate of heat transfer, the exit temperature of exhaust gases, and the rate of evaporation of water. Let the temperature of exhaust gases vary from 300°C to 600°C. Plot the rate of heat transfer, the exit temperature of exhaust gases, and the rate of evaporation of water as a function of the temperature of the exhaust gases, and discuss the results.  

Under steady-state conditions, air at a temperature of 20.0°C, pressure of 1.00 atm, and a velocity of 18.5 m/sec flows over the top surface of a flat-plate heater that is kept at a temperature of 135.0°C. The heater is a circular disk with a diameter of 0.50 meters. The air flowing over the top surface of the disk creates a drag force of 0.25 Newtons.   Using the modified Reynolds analogy, calculate the heat transfer rate from the top surface of the plate heater.

A 15 cm x 15 cm circuit board dissipating 15 W of power uniformly is cooled by air, which approaches the circuit board at 20°C with a velocity of 5 m/s. Disregard any heat transfer from the back surface of the board and evaluate properties of air at 300 K. Assume the flow to be turbulent across the entire length of the board since the electric components will have tripped the boundary layer. (a) Determine the surface temperature of the electronic components at the leading edge of the board, and (b) at the end of the board. (c) Ideally, at what temperature would the properties of air be evaluated at?

For safety reasons, parts can be directly blown by air. with dimensions of 15 cmx20 cm, which are not allowed to contact printed circuit board, 20 cm long 0.2 opened inside Cold air from a rectangular hole of cmx14 cm will be cooled. from electronic parts The heat generated is transmitted from the thin layer of the card to the duct, where it is combined with the air entering the duct at a temperature of 15 °C. is removed. The heat flux on the upper surface of the channel can be considered uniform and The heat transfer from the surfaces can be neglected. If the velocity of the air in the duct does not exceed (590) m/min and the surface temperature of the duct is constant at 50 °C, this circuit board can be safely placed on it. Calculate the maximum total power of the electronic parts to be placed.

A metallic airfoil of elliptical cross section has a mass of 50 kg, surface area of 12 m2, and a specific heat of 0.50 kJ/kg K. The airfoil is subjected to air flow at 1 atm, 25°C, and 5 m/s along its 3-m-long side. The average temperature of the airfoil is observed to drop from 160°C to 150°C within 2 min of cooling. Assuming the surface temperature of the airfoil to be equal to its average temperature and using momentum-heat transfer analogy, determine the average friction coefficient of the airfoil surface. Answer: 0.000363

Required information Air flows in a pipe under fully developed conditions with an average velocity of 1.25 m/s and a temperature of 24°C. The pipe's inner diameter is 4 cm, and its length is 4 m. The first half of the pipe is kept at a constant wall temperature of 100°C. The second half of the pipe is subjected to a constant heat flux of 200 W. The properties of air at 80°C are p = 0.9994 kg/m³, k = 0.02953 W/m-K, v= 2.097 x 10-5 m²/s, cp=1008 J/kg-K, and Pr = 0.7154. NOTE: This is a multi-part question. Once an answer is submitted, you will be unable to return to this part. Air 1.25 m/s 2m T, = 100°C D = 4 cm 2 m g, = 200 W Determine the wall temperature at the exit of the tube. The wall temperature at the exit of the tube is 282 * °C.