Problem 65
A \(15-\mathrm{cm} \times 15-\mathrm{cm}\) circuit board dissipating $20 \mathrm{~W}$ of power uniformly is cooled by air, which approaches the circuit board at \(20^{\circ} \mathrm{C}\) with a velocity of $6 \mathrm{~m} / \mathrm{s}$. Disregarding any heat transfer from the back surface of the board, determine the surface temperature of the electronic components \((a)\) at the leading edge and \((b)\) at the end of the board. Assume the flow to be turbulent since the electronic components are expected to act as turbulators. For air properties evaluations, assume a film temperature of $35^{\circ} \mathrm{C}$. Is this a good assumption?
Problem 66
In flow over blunt bodies such as a cylinder, how does the pressure drag differ from the friction drag?
Problem 67
In flow over cylinders, why does the drag coefficient suddenly drop when the flow becomes turbulent? Isn't turbulence supposed to increase the drag coefficient instead of decreasing it?
Problem 68
Why is flow separation in flow over cylinders delayed in turbulent flow?
Problem 69
Consider laminar flow of air across a hot circular cylinder. At what point on the cylinder will the heat transfer be highest? What would your answer be if the flow were turbulent?
Problem 71
A long 12-cm-diameter steam pipe whose external surface temperature is \(90^{\circ} \mathrm{C}\) passes through some open area that is not protected against the winds. Determine the rate of heat loss from the pipe per unit of its length when the air is at \(1 \mathrm{~atm}\) pressure and $7^{\circ} \mathrm{C}\( and the wind is blowing across the pipe at a velocity of \)65 \mathrm{~km} / \mathrm{h}$.
Problem 72
Two metal plates are connected by a long ASTM B98 copper-silicon bolt. Air, at \(250^{\circ} \mathrm{C}\), flows at \(17 \mathrm{~m} / \mathrm{s}\) between the plates and across the cylindrical bolt. The diameter of the bolt is $9.5 \mathrm{~mm}\(, and the length of the bolt exposed to the air is \)10 \mathrm{~cm}$. The maximum use temperature for the ASTM B98 bolt is \(149^{\circ} \mathrm{C}\) (ASME Code for Process Piping, ASME B31.3-2014, Table A-2M). The temperature of the bolt is maintained by a cooling mechanism with the capability of removing heat at a rate of \(30 \mathrm{~W}\). Determine whether the heat removed from the bolt is sufficient to keep the bolt at \(149^{\circ} \mathrm{C}\) or lower.
Problem 73
A heated long 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 its surface has an emissivity of \(0.95\). If the surrounding temperature is \(20^{\circ} \mathrm{C}\) and the heat flux dissipated from the rod is \(16,000 \mathrm{~W} / \mathrm{m}^{2}\), determine the surface temperature of the rod. Evaluate the air properties at \(70^{\circ} \mathrm{C}\).
Problem 76
In a geothermal power plant, the used geothermal water at $80^{\circ} \mathrm{C}$ enters a 15 -cm-diameter and 400 -m-long uninsulated pipe at a rate of \(8.5 \mathrm{~kg} / \mathrm{s}\) and leaves at \(70^{\circ} \mathrm{C}\) before being reinjected back to the ground. Windy air at $15^{\circ} \mathrm{C}$ flows normal to the pipe. Disregarding radiation, determine the average wind velocity in \(\mathrm{km} / \mathrm{h}\).
Problem 77
In a piece of cryogenic equipment, two metal plates are connected by a long ASTM A437 B4B stainless steel bolt. Cold gas, at \(-70^{\circ} \mathrm{C}\), flows between the plates and across the cylindrical bolt. The gas has a thermal conductivity of \(0.02 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\), a kinematic viscosity of \(9.3 \times 10^{-6} \mathrm{~m}^{2} / \mathrm{s}\), and a Prandtl number of \(0.74\). The diameter of the bolt is \(9.5 \mathrm{~mm}\), and the length of the bolt exposed to the gas is \(10 \mathrm{~cm}\). The minimum temperature suitable for the ASTM A437 B4B stainless steel bolt is \(-30^{\circ} \mathrm{C}\) (ASME Code for Process Piping, ASME B31.3-2014, Table A-2M). The temperature of the bolt is maintained by a heating mechanism capable of providing heat at \(15 \mathrm{~W}\). Determine the maximum velocity that the gas can achieve without cooling the bolt below the minimum suitable temperature of \(-30^{\circ} \mathrm{C}\).
