Problem 122
Oil at \(60^{\circ} \mathrm{C}\) flows at a velocity of $20 \mathrm{~cm} / \mathrm{s}\( over a \)5.0\(-m-long and \)1.0$-m-wide flat plate maintained at a constant temperature of \(20^{\circ} \mathrm{C}\). Determine the rate of heat transfer from the oil to the plate if the average oil properties are $\rho=880 \mathrm{~kg} / \mathrm{m}^{3}\(, \)\mu=0.005 \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}, k=0.15 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\(, and \)c_{p}=2.0 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K}$.
Problem 124
A thin, square, flat plate has \(1.2 \mathrm{~m}\) on each side. Air at \(10^{\circ} \mathrm{C}\) flows over the top and bottom surfaces of a very rough plate in a direction parallel to one edge, with a velocity of $48 \mathrm{~m} / \mathrm{s}$. The surface of the plate is maintained at a constant temperature of \(54^{\circ} \mathrm{C}\). The plate is mounted on a scale that measures a drag force of \(1.5 \mathrm{~N}\). Determine the total heat transfer rate from the plate to the air.
Problem 125
Consider a house that is maintained at a constant temperature of $22^{\circ} \mathrm{C}$. One of the walls of the house has three single-pane glass windows that are \(1.5 \mathrm{~m}\) high and \(1.8 \mathrm{~m}\) long. The glass $(k=0.78 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\( is \)0.5 \mathrm{~cm}$ thick, and the heat transfer coefficient on the inner surface of the glass is $8 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\(. Now winds at \)35 \mathrm{~km} / \mathrm{h}$ start to blow parallel to the surface of this wall. If the air temperature outside is \(-2^{\circ} \mathrm{C}\), determine the rate of heat loss through the windows of this wall. Assume radiation heat transfer to be negligible. Evaluate the air properties at a film temperature of $5^{\circ} \mathrm{C}\( and \)1 \mathrm{~atm}$.
Problem 126
An automotive engine can be approximated as a \(0.4\)-m-high, \(0.60\)-m-wide, and \(0.7-\mathrm{m}\)-long rectangular block. The bottom surface of the block is at a temperature of \(75^{\circ} \mathrm{C}\) and has an emissivity of \(0.92\). The ambient air is at \(5^{\circ} \mathrm{C}\), and the road surface is at \(10^{\circ} \mathrm{C}\). Determine the rate of heat transfer from the bottom surface of the engine block by convection and radiation as the car travels at a velocity of \(60 \mathrm{~km} / \mathrm{h}\). Assume the flow to be turbulent over the entire surface because of the constant agitation of the engine block. How will the heat transfer be affected when a 2 -mm-thick layer of gunk $(k=3 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})$ has formed at the bottom surface as a result of the dirt and oil collected at that surface over time? Assume the metal temperature under the gunk is still \(75^{\circ} \mathrm{C}\).
Problem 127
A 15 -ft-long strip of sheet metal is being transported on a conveyor at a velocity of \(16 \mathrm{ft} / \mathrm{s}\). To cure the coating on the upper surface of the sheet metal, infrared lamps providing a constant radiant flux of \(1500 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft}^{2}\) are used. The coating on the upper surface of the metal strip has an absorptivity of \(0.6\) and an emissivity of \(0.7\), while the surrounding ambient air temperature is \(77^{\circ} \mathrm{F}\). Radiation heat transfer occurs only on the upper surface, while convection heat transfer occurs on both upper and lower surfaces of the sheet metal. Determine the surface temperature of the sheet metal. Evaluate the properties of air at \(180^{\circ} \mathrm{F}\).
Problem 128
To defrost ice accumulated on the outer surface of an automobile windshield, warm air is blown over the inner surface of the windshield. Consider an automobile windshield $\left(k_{w}=1.4 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\right)\( with an overall height of \)0.5 \mathrm{~m}$ and thickness of \(5 \mathrm{~mm}\). The outside air (1 atm) ambient temperature is \(-20^{\circ} \mathrm{C}\), and the average airflow velocity over the outer windshield surface is \(80 \mathrm{~km} / \mathrm{h}\), while the ambient temperature inside the automobile is \(25^{\circ} \mathrm{C}\). Determine the value of the convection heat transfer coefficient for the warm air blowing over the inner surface of the windshield that is needed to cause the accumulated ice to begin melting. Assume the windshield surface can be treated as a flat plate surface.
Problem 129
The local atmospheric pressure in Denver, Colorado (elevation $1610 \mathrm{~m}\( ), is \)83.4 \mathrm{kPa}\(. Air at this pressure and \)20^{\circ} \mathrm{C}\( flows with a velocity of \)8 \mathrm{~m} / \mathrm{s}$ over a \(1.5-\mathrm{m} \times 6-\mathrm{m}\) flat plate whose temperature is \(140^{\circ} \mathrm{C}\). Determine the rate of heat transfer from the plate if the air flows parallel to \((a)\) the 6 -m-long side and \((b)\) the \(1.5-\mathrm{m}\) side.
Problem 132
In the effort to increase the removal of heat from a hot surface at \(120^{\circ} \mathrm{C}\), a cylindrical pin fin $\left(k_{f}=237 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\right)\( with a diameter of \)5 \mathrm{~mm}$ is attached to the hot surface. Air at \(20^{\circ} \mathrm{C}\) (1 atm) is flowing across the pin fin with a velocity of \(10 \mathrm{~m} / \mathrm{s}\).
Problem 135
Steam at \(250^{\circ} \mathrm{C}\) flows in a stainless steel pipe $(k=15 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})$ whose inner and outer diameters are \(4 \mathrm{~cm}\) and \(4.6 \mathrm{~cm}\), respectively. The pipe is covered with \(3.5-\mathrm{cm}\)-thick glass wool insulation $(k=0.038 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\( whose outer surface has an emissivity of \)0.3$. Heat is lost to the surrounding air and surfaces at \(3^{\circ} \mathrm{C}\) by convection and radiation. Taking the heat transfer coefficient inside the pipe to be \(80 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the rate of heat loss from the steam per unit length of the pipe when air is flowing across the pipe at \(4 \mathrm{~m} / \mathrm{s}\). Evaluate the air properties at a film temperature of \(10^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\).
Problem 136
A cylindrical rod is placed in a crossflow of air at $20^{\circ} \mathrm{C}(1 \mathrm{~atm})\( with velocity of \)10 \mathrm{~m} / \mathrm{s}$. The rod has a diameter of \(5 \mathrm{~mm}\) and a constant surface temperature of \(120^{\circ} \mathrm{C}\). Determine \((a)\) the average drag coefficient, \((b)\) the convection heat transfer coefficient using the Churchill and Bernstein relation, and (c) the convection heat transfer coefficient using Table 7-1.
