Problem 125
What is the relation \((f / 2) \operatorname{Re}=\mathrm{Nu}=\) Sh known as? Under what conditions is it valid? What is the practical importance of it?
Problem 126
What is the name of the relation \(f 72=\mathrm{St} \operatorname{Pr}^{2 / 3}=\) \(\mathrm{St}_{\text {mass }} \mathrm{Sc}^{2 / 3}\), and what are the names of the variables in it? Under what conditions is it valid? What is the importance of it in engineering?
Problem 127
Using the analogy between heat and mass transfer, explain how the mass transfer coefficient can be determined from the relations for the heat transfer coefficient.
Problem 128
What is the relation \(h_{\text {heat }}=\rho c_{p} h_{\text {mass }}\) known as? For what kind of mixtures is it valid? What is the practical importance of it?
Problem 129
What is the low mass flux approximation in mass transfer analysis? Can the evaporation of water from a lake be treated as a low mass flux process?
Problem 131
Consider a wet concrete patio covered with a thin film of water. At the surface, mass convection of water to air occurs at an average mass transfer coefficient of \(0.03 \mathrm{~m} / \mathrm{s}\). If the air is at $1 \mathrm{~atm}, 15^{\circ} \mathrm{C}$ and 35 percent relative humidity, determine the mass fraction concentration gradient of water at the surface.
Problem 133
The average heat transfer coefficient for airflow over an odd-shaped body is to be determined by mass transfer measurements and using the Chilton-Colburn analogy between heat and mass transfer. The experiment is conducted by blowing dry air at \(1 \mathrm{~atm}\) at a free-stream velocity of $2 \mathrm{~m} / \mathrm{s}$ over a body covered with a layer of naphthalene. The surface area of the body is \(0.75 \mathrm{~m}^{2}\), and it is observed that $100 \mathrm{~g}\( of naphthalene has sublimated in \)45 \mathrm{~min}$. During the experiment, both the body and the air were kept at \(25^{\circ} \mathrm{C}\), at which the vapor pressure and mass diffusivity of naphthalene are $11 \mathrm{~Pa}\( and \)D_{A B}=0.61 \times 10^{-5} \mathrm{~m}^{2} / \mathrm{s}$, respectively. Determine the heat transfer coefficient under the same flow conditions over the same geometry.
Problem 134
The local convection heat transfer coefficient for air flowing parallel over a 1 -m-long plate with irregular surface topology is experimentally determined to be \(h_{x}=0.5+12 x-0.7 x^{3}\), where \(h_{x}\) is in $\mathrm{W} / \mathrm{m}^{2} \cdot \mathrm{K}$. If the plate surface is coated with water, determine the corresponding average mass convection coefficient over the entire plate. Assume properties can be evaluated at \(298 \mathrm{~K}\) and $1 \mathrm{~atm}$.
Problem 137
A thin slab of solid salt (NaCl) with dimensions of $0.15 \mathrm{~m} \times 0.15 \mathrm{~m}\( is being dragged through seawater \)\left(\nu=1.022 \times 10^{-6} \mathrm{~m}^{2} / \mathrm{s}\right)$ at an average relative velocity of \(0.6 \mathrm{~m} / \mathrm{s}\). The seawater at \(18^{\circ} \mathrm{C}\) has a salt concentration of \(31 \mathrm{~kg} / \mathrm{m}^{3}\), while the salt slab has a concentration of \(35,000 \mathrm{~kg} / \mathrm{m}^{3}\). If the diffusion coefficient of salt in seawater is $1.2 \times 10^{-9} \mathrm{~m}^{2} / \mathrm{s}$, determine the mass convection rate of salt being dissolved in the seawater.
Problem 141
Consider a \(5-\mathrm{m} \times 5-\mathrm{m}\) wet concrete patio with an average water film thickness of \(0.2 \mathrm{~mm}\). Now wind at $50 \mathrm{~km} / \mathrm{h}\( is blowing over the surface. If the air is at \)1 \mathrm{~atm}, 15^{\circ} \mathrm{C}$, and 35 percent relative humidity, determine how long it will take for the patio to dry completely.
