Chapter 10

Consider a two phase flow of air-water in a vertical upward stainless steel pipe with an inside diameter of $0.0254\mathrm{m}$. The two phase mixture enters the pipe at ${25}^{\circ }\mathrm{C}$ at a system pressure of $201\mathrm{kPa}$. The superficial velocities of the water and air are $0.3\mathrm{m}/\mathrm{s}$ and $23\mathrm{m}/\mathrm{s}$, respectively. The differential pressure transducer connected across the pressure taps set $1\mathrm{m}$ apart records a pressure drop of $2700\mathrm{Pa}$ and the measured value of void fraction is $0.86$. Using the concept of Reynolds analogy determine the two phase convective heat transfer coefficient. Hint: Use EES to calculate the properties of water and air at the given temperature and pressure.

Water is to be boiled at sea level in a 30 -cm-diameter mechanically polished AISI 304 stainless steel pan placed on top of a $3-\mathrm{kW}$ electric burner. If 60 percent of the heat generated by the burner is transferred to the water during boiling, determine the temperature of the inner surface of the bottom of the pan. Also, determine the temperature difference between the inner and outer surfaces of the bottom of the pan if it is 6-mm thick. Assume the boiling regime is nucleate boiling. Is this a good assumption?

Saturated ammonia vapor at ${25}^{\circ }\mathrm{C}$ condenses on the outside of a 2 -m-long, 3.2-cm-outer-diameter vertical tube maintained at ${15}^{\circ }\mathrm{C}$. Determine $(a)$ the average heat transfer coefficient, $(b)$ the rate of heat transfer, and $(c)$ the rate of condensation of ammonia. Assume turbulent flow and that the tube diameter is large, relative to the thickness of the liquid film at the bottom of the tube. Are these good assumptions?

When boiling a saturated liquid, one must be careful while increasing the heat flux to avoid burnout. Burnout occurs when the boiling transitions from boiling. (a) convection to nucleate (b) convection to film (c) film to nucleate (d) nucleate to film (e) none of them

At a distance $x$ down a vertical, isothermal flat plate on which a saturated vapor is condensing in a continuous film, the thickness of the liquid condensate layer is $\delta$. The heat transfer coefficient at this location on the plate is given by (a) $k_l/\delta$ (b) $\delta h_f$ (c) $\delta h_{fg}$ (d) $\delta h_g$ (e) none of them

When a saturated vapor condenses on a vertical, isothermal flat plate in a continuous film, the rate of heat transfer is proportional to (a) ${(T_s-T_{\text{sat }})}^{1/4}$ (b) ${(T_s-T_{\mathrm{sat}})}^{1/2}$ (c) ${(T_s-T_{\text{sat }})}^{3/4}$ (d) $(T_s-T_{\text{sat }})$ (e) ${(T_s-T_{\mathrm{sat}})}^{2/3}$

Saturated water vapor is condensing on a $0.5{\mathrm{m}}^2$ vertical flat plate in a continuous film with an average heat transfer coefficient of $7\mathrm{kW}/{\mathrm{m}}^2\cdot \mathrm{K}$. The temperature of the water is ${80}^{\circ }\mathrm{C}(h_{fg}=2309\mathrm{kJ}/\mathrm{kg})$ and the temperature of the plate is ${60}^{\circ }\mathrm{C}$. The rate at which condensate is being formed is (a) $0.0303\mathrm{kg}/\mathrm{s}$ (b) $0.07\mathrm{kg}/\mathrm{s}$ (c) $0.15\mathrm{kg}/\mathrm{s}$ (d) $0.24\mathrm{kg}/\mathrm{s}$ (e) $0.28\mathrm{kg}/\mathrm{s}$

Steam condenses at ${50}^{\circ }\mathrm{C}$ on a $0.8-\mathrm{m}$-high and $2.4-\mathrm{m}-$ wide vertical plate that is maintained at ${30}^{\circ }\mathrm{C}$. The condensation heat transfer coefficient is (a) $3975\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (b) $5150\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (c) $8060\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (d) $11,300\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (e) $14,810\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (For water, use ${\rho }_l=992.1\mathrm{kg}/{\mathrm{m}}^3,{\mu }_l=0.653\times {10}^{-3}\mathrm{kg}/\mathrm{m}\cdot \mathrm{s}$, $k_l=0.631\mathrm{W}/\mathrm{m}\cdot \mathrm{K},c_{pl}=4179\mathrm{J}/\mathrm{kg}\cdot {}^{\circ }\mathrm{C},h_{fg\oplus T_{\text{sat }}}=2383\mathrm{kJ}/\mathrm{kg})$

An air conditioner condenser in an automobile consists of $2{\mathrm{m}}^2$ of tubular heat exchange area whose surface temperature is ${30}^{\circ }\mathrm{C}$. Saturated refrigerant-134a vapor at ${50}^{\circ }\mathrm{C}$ $(h_{fg}=152\mathrm{kJ}/\mathrm{kg})$ condenses on these tubes. What heat transfer coefficent must exist between the tube surface and condensing vapor to produce $1.5\mathrm{kg}/\min$ of condensate? (a) $95\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (b) $640\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (c) $727\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (d) $799\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$ (e) $960\mathrm{W}/{\mathrm{m}}^2\cdot \mathrm{K}$

Saturated water vapor at ${40}^{\circ }\mathrm{C}$ is to be condensed as it flows through a tube at a rate of $0.2\mathrm{kg}/\mathrm{s}$. The condensate leaves the tube as a saturated liquid at ${40}^{\circ }\mathrm{C}$. The rate of heat transfer from the tube is (a) $34\mathrm{kJ}/\mathrm{s}$ (b) $268\mathrm{kJ}/\mathrm{s}$ (c) $453\mathrm{kJ}/\mathrm{s}$ (d) $481\mathrm{kJ}/\mathrm{s}$ (e) $515\mathrm{kJ}/\mathrm{s}$