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engineering
chemical engineering
Questions and Answers of
Chemical Engineering
Show all the steps leading to the integral balance equation (18.6) in the text.Use the following boundary conditions and verify Eq. (18.7). (i) At \(y=0, T=T_{\mathrm{S}}\). (ii) At
For fluids with \(\operatorname{Pr}
Consider the flat-plate heat transfer to be solved by the similarity approach. Show all the details leading to Eq. (18.15). What are the boundary conditions on \(\theta\) ? Extend your MATLAB code to
For the similarity solution, what are the boundary conditions for the constant-wall-flux case? Show that a complete similarity does not exist for this case. Also show the condition for the case where
Air at \(300 \mathrm{~K}\) and \(1 \mathrm{~atm}\) flows along a flat plate at \(3 \mathrm{~m} / \mathrm{s}\). At a location of \(0.3 \mathrm{~m}\) from the leading edge, find the thickness of the
Extend the analysis of heat transfer over a wedge flow. Derive the following equation for the temperature profile:\[\begin{equation*}\theta^{\prime \prime}+(m+1) \operatorname{Prf} \theta^{\prime}=0
Consider heat transfer over a flat plate again but now include an additional term due to viscous heating. Show that the similarity method is applicable to this problem as well, and derive the
Integral balances can also be used for heat transfer in a turbulent-flow boundary layer if a form for the velocity profile is assumed. A common form is the 1/7th-power
A vertical plate \(3 \mathrm{~m}\) long is at a temperature of \(400 \mathrm{~K}\) and exposed to air at \(300 \mathrm{~K}\). Calculate the thickness of the boundary layer and the value of the local
An immersion heater operating at \(1000 \mathrm{~W}\) is in the form of a rectangular solid with dimensions of \(16 \mathrm{~cm}\) by \(10 \mathrm{~cm}\) by \(1 \mathrm{~cm}\). Determine the heat
Water is boiled in a polished stainless steel pot with a \(3 \mathrm{~kW}\) heater. The efficiency of the heater is \(60 \%\), i.e., only \(60 \%\) of the heat is transferred to the water. Find the
Consider boiling of water under sea-level conditions in a copper vessel. Calculate and plot the heat flux vs. \(\Delta T\) diagram for water at three different pressures. Show the temperature at the
Consider a film of vapor in contact with a liquid. From a heat balance show that the mass flow rate in the vapor, \(\dot{m}\), per unit transfer area changes as\[\hat{h}_{\lg } \frac{d \dot{m}}{d
Find the temperature at which the surface tension of water becomes zero. What is the physical significance of this temperature?
Consider a vapor condensing on a wall and forming a liquid film. Assume that locally the film thickness is related to the flow rate per unit transfer area by momentum-transfer considerations.Then
Saturated steam at \(356 \mathrm{~K}\) condenses on a vertical tube of diameter \(5 \mathrm{~cm}\) whose surface is maintained at \(340 \mathrm{~K}\).Find the height at which the flow becomes wavy.
Saturated steam at \(55^{\circ} \mathrm{C}\) is to be condensed at a rate of \(10 \mathrm{~kg} / \mathrm{h}\) on the outside of a vertical tube of diameter \(3 \mathrm{~cm}\) by maintaining the
What are the key dimensionless parameters? Integrate the equation to find an expression for \(\lambda / L\) as a function of dimensionless time. Is a steady state reached in this system?
A deep pool of water is initially at \(4^{\circ} \mathrm{C}\) and suddenly the surface is exposed to a freezing front at \(-4{ }^{\circ} \mathrm{C}\) and remains so exposed for a long time.Find the
A common procedure in metallurgy is solidification from a melt in a cylindrical mold. If the Jacob number is small we can use the pseudo-steady-state model, which should now include the conduction in
A stainless-steel component is exposed to laser heating at an initial temperature of \(300 \mathrm{~K}\). After a short transient the surface reaches its melting point, and the surface recedes at a
A schematic diagram of crystal growth by directional solidification is shown in Fig. 18.8. Analyze the various flow mechanisms and indicate how they affect the heat transfer, the movement of the
A heat pipe is an evaporator-condenser system in which the liquid is returned to the evaporator by capillary action. In the simplest form it consists of a wire-mesh region that serves to act as a
Using the basic physical constants, show that \(C_{1}=3.742 \times 10^{8} \mathrm{~W} \mu \mathrm{m}^{4} / \mathrm{m}^{2}\) and \(C_{2}=\) \(1.4389 \times 10^{4} \mu \mathrm{m} / \mathrm{K}\) in the
The solar constant is defined as the average flux of solar energy incident on the outer fringes of the Earth's atmosphere and a commonly used value is \(1.353 \mathrm{~kW} / \mathrm{m}^{2}\). If the
Verify the algebra leading to Eq. (19.22) for radiation exchange between two plates facing each other. 1 21 -(Eb1 - Eb2) (19.22) 1/1 + 1/62-1
Consider a shield with an emissivity of \(\epsilon_{\mathrm{s}}\) separating two parallel plates as shown in Fig. 19.11. Assume that the shield is adiabatic and that there is no net loss of heat.
Consider radiation exchange between two annular cylinders of radii \(R_{\mathrm{i}}\), and \(R_{\mathrm{o}}\) facing each other. This is similar to the radiation between flat plates considered in the
Two parallel disks each of radius \(R\) are facing each other and separated by a distance \(H\). Derive an expression for the view factor between these disks.
Two rectangular plates of sizes \(L\) and \(W\) are facing each other and separated by a distance \(H\). Derive an expression for the view factor between these disks. Find the limit if \(W\) is
Consider two plates at temperatures of \(600 \mathrm{~K}\) and \(500 \mathrm{~K}\) with emissivities of 0.8 and 0.4 , respectively. The plates are separated by a gray gas that has an absorption
A solid at an initial temperature of \(T_{0}\) is exposed to colder surroundings at \(T_{\mathrm{a}}\) and is cooling down by radiation heat loss. Develop a model based on the lumping approximation
A gray gas with an absorption coefficient of \(\kappa\) is contained between two plates separated by a distance \(L\). Using the diffusion approximation for radiation, derive the
Two plates are at temperatures of \(T_{1}\) and \(T_{2}\) and a chemical reaction is producing heat at a constant rate within the system. Derive a model to predict the temperature distribution within
The common methods for the computational solution of radiative transport equations are summarized briefly below:1. The discrete ordinate method 2. The Monte-Carlo method 3. Hottel's zonal
The calculation of solar radiation impinging on a surface is of importance in many applications, for example, design of solar collectors, temperature control of buildings, etc.Your goal is to review
Radiation interactions are of importance in crystal pullers for growing crystals from melts since these operate at very high temperatures. The radiation view factors change at various stages of
Although radiation is important in heat transfer, an analogous model can be used in the design of photochemical reactors. The modeling of these reactors requires that the radiation intensity be
Develop an expression for the average mass transfer coefficient for a plate of length \(L\), where part of the flow is turbulent for part of the plate. Assume that there is no transition and that the
The factor \(j_{\mathrm{D}}\) is also used in the correlation. This is defined as\[j_{\mathrm{D}}=S t(S c)^{2 / 3}=\frac{k_{\mathrm{m}}}{v_{\text {ref }}}(S c)^{2 / 3}\]where \(S t\) is the Stanton
Verify that the \(j\)-factor is related to the drag coefficient by the relation\[j_{\mathrm{D}}=\frac{c_{\mathrm{D}}}{2}\]for mass transfer for flow over a flat plate.
A silicon substrate \(10 \mathrm{~cm}\) long is exposed to a gas stream containing an arsenic precursor so that a GaAs film can be deposited on the surface. Estimate the mass-transfer coefficient,
The scaling analysis for the mass-transfer boundary-layer thickness is parallel to that for heat transfer analysis. Use the same method to show that \(\Delta\) is proportional to \(S c^{-1 / 3}\) for
An indirect way of measuring of secondary emission from ponds or large bodies of water used in waste treatment is to measure the concentration and velocity over the surface. The data can then be
Chemical vapor deposition (CVD) on an inclined susceptor: a case-study problem. An important application of convective mass transfer theory is in CVD processes employed to coat surfaces with thin
Mass transfer from a bubble. Calculate the mass transfer coefficient for the air-water system for bubbles rising at a gas velocity of \(5 \mathrm{~cm} / \mathrm{s}\) in a pool of stagnant liquid. Use
Axial dispersion in channel flow. Consider the pressure-driven laminar flow in a channel of height \(2 h\). Derive the following formula for the axial dispersion coefficient:\[D_{\mathrm{E}}=\frac{2
Axial dispersion in turbulent flow. Taylor showed that the following expression is suitable for the calculation of the axial dispersion coefficient for turbulent flow in a pipe:\[D_{\mathrm{E}}=10 R
A model for a hemodialyser with simulation of the patient-artificial-kidney system: a case-study problem. A useful case study is the paper by Ramachandran and Mashelkar (1980), where a mesoscopic
A model for chromatographic separation: a case-study problem. An important application of Taylor dispersion is in chromatography. Here pulses of a mixture of solutes are introduced into one end of a
Show that the two representations of the Stefan-Maxwell model given by Eqs. (21.4) and (21.5) are equivalent. ns -VA = j=1 y;Ni-yiNj CDij (21.4)
Show that the Stefan-Maxwell model can be rearranged to define a pseudo-binary diffusivity, \(D_{i \mathrm{~m}}\), for species \(i\) in the mixture:\[\frac{1}{D_{i
Support for the form of the generalized version of Fick's law introduced in this chapter can be found in the thermodynamics of irreversible processes. Here we introduce a brief description of this
Acetone is evaporating in a mixture of nitrogen and helium. Find the rate of evaporation and compare it with the rates in pure nitrogen and pure helium. Also compare it with the model using a
Catalytic oxidation of \(\mathrm{CO}\) is an important reaction in pollution prevention. The reaction scheme is\[\mathrm{O}_{2}(\mathrm{~A})+2 \mathrm{CO}(\mathrm{B}) \rightarrow 2
Examine how the rank of the stoichiometric matrix can be used to reduce the number of equations to be solved. For example, if there are \(n_{\mathrm{s}}\) components and \(n_{\mathrm{r}}\) equations
Extend the analysis two reactions, making it applicable for steam reforming of methane. The first reaction is\[\mathrm{CH}_{4}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CO}+3
Consider the three-component system consisting of acetaldehyde (1), hydrogen (2), and ethanol (3). The binary diffusivity values at \(548 \mathrm{~K}\) and \(101.3 \mathrm{kPa}\) are given in Example
In the matrix representation the species \(n\) chosen for elimination is usually referred to as the solvent. The choice of "solvent" species is arbitrary, but it can have an effect on the coefficient
Derive the solutions for transient concentration profiles in the two-bulb apparatus (Example 21.5 in the text) for the binary case, and show that the multicomponent case can be derived as an
When two gases A and B are forced to diffuse through a third gas C, there is a tendency of A and \(\mathrm{B}\) to separate because of the difference in their diffusivities in gas \(\mathrm{C}\).
Show that another way of writing the flux due to pressure diffusion is in terms of partial molar volume and the total mixture density is\[J_{\mathrm{A}}^{*}(\text { pressure diffusion })=-C
Estimate the steady-state concentration profile when a typical albumin solution is subjected to a centrifugal field of 30000 times the force of gravity under the following conditions: the cell length
A mixture of \(\mathrm{H}_{2}\) and \(\mathrm{D}_{2}\) is contained in two bulbs connected by a porous plug. The bulbs are maintained at different temperatures of 200 and \(600 \mathrm{~K}\). The
Verify that both the pressure and the vorticity field satisfy the Laplace equation for Stokes flow.Verify that the velocity field satisfies the biharmonic equation\[\begin{equation*}abla^{4}
Show that the force acting on a control surface of any arbitrary control volume is equal to the force on a larger regularly shaped control volume enclosing the given body. In order to do this you
Confirm that the definition of the streamfunction given in the text satisfies the continuity equation.Use this in the N-S equation and derive the form for the \(E^{4}\) operator in spherical
Consider the flow in cylindrical coordinates with no dependence on \(\theta\). How is the streamfunction defined? What is the non-vanishing component of vorticity. Show that the \(E^{2}\) operator
Derive the general solution for \(\psi\) given in the text (Eq. (15.9)). 0 (r,n) = A++ Bnr "+1 + Cnr -" + Dnr"1Qn(n) 2-n (15.9) n=1
What are units for \(C_{1}\) in Eq. (15.10)? Confirm that the units agree on both sides. F = 4 (15.10)
A sphere of radius \(R\) is rotating in an infinite fluid at an angular velocity of \(\Omega\). Derive the following expression for the velocity field:\[v_{\phi}=r \Omega \sin \theta
Find the solution to Stokes flow past a sphere where the far-field velocity satisfies the elongational flow defined as \(v_{x}=\dot{\gamma} x, v_{y}=\) \(\dot{\gamma} y\), and \(v_{z}=-2 \dot{\gamma}
The general solution to Stokes flow in 2D Cartesian coordinates. For the 2D case the governing equation is \(abla^{4} \psi=0\). The operator \(abla\) may be applied either in Cartesian \((x, y)\) or
The case of flow past a cylinder of infinite length normal to the axis was also studied by Stokes. In view of the 2D nature of the problem, it is more convenient to work in \(r, \theta\) coordinates.
Verify the vector identity\[\begin{equation*}abla^{2} \boldsymbol{v}=abla(abla \cdot \boldsymbol{v})-abla \times(abla \times \boldsymbol{v}) \tag{15.83}\end{equation*}\]
Show that, for irrotational axisymmetric flows in cylindrical coordinates, the streamfunction satisfies the following equation:\[\frac{\partial^{2} \psi}{\partial r^{2}}-\frac{1}{r}
Indicate the type of flow given by \(\phi=\sqrt{r} \cos (\theta / 2)\). Calculate and plot typical streamlines.
Consider a source of flow in \(3 \mathrm{D}\) of strength \(S\) (dimensions \(L^{3} / T\) ), Then the flow is axisymmetric and independent of the \(\theta\)-direction in the spherical coordinate
Verify the principle of superposition, which states that if \(\phi_{1}\) and \(\phi_{2}\) are solutions to potential flow then \(\phi=c_{1} \phi_{1}+c_{2} \phi_{2}\) is also a solution.Also show that
Verify by direct substitution that the functions \(r^{n} \cos (n \theta)\) etc. shown in the text satisfy the Laplace equation in polar coordinates.Calculate the streamfunction for the
State the form of the Laplace equation in axisymmetric spherical coordinates.Verify that the following functions satisfy this equation:\[r \cos \theta ; \quad \cos \theta / r^{2}\]A linear
A cylinder of diameter \(1.2 \mathrm{~m}\) and length \(7.5 \mathrm{~m}\) rotates at \(90 \mathrm{r}\).p.m. with its axis perpendicular to an air stream with an approach velocity of \(3.6 \mathrm{~m}
Model equations similar to those for potential flow arise in flow in porous media, which has a wide variety of applications, e.g., in groundwater treatment, water-purity remediation, filtration, flow
One way of cleaning up environmentally polluted underground water is to pump it out of the ground, treat it with some equipment (by catalytic or photochemical oxidation, for example), and then pump
Consider the flow past a flat plate. Here we examine the effect of a superimposed velocity \(v_{y 0}\) at the plate surface. This can be done by the integral method with only minor changes in the
Find the value of \(m\) for which the wall shear stress is independent of the principal flow direction.Find the value of \(m\) for which the boundary-layer thickness is a constant.
Consider the problem of a semi-infinite fluid subject to a constant shear at the interface. This can be caused, for instance, by a surface-tension gradient. Show that the following differential
Von Kármán assumed a cubic profile for the integral momentum analysis over a flat plate. Since a cubic has four constants, four conditions were used.(i) \(V_{x}=0\) at \(y=0\).(ii)
Consider a boundary layer with no external pressure gradient (a flat plate). Then, from the Prandtl boundary-layer equation, deduce that\[\mu \frac{d^{2} v_{x}}{d y^{2}}=0 \text { at } y=0\]Now
A cubic approximation is commonly used in conjunction with the von Kármán momentum integral. An alternative form is the sine function:\[v_{x}=\alpha \sin (b y)\]What should the constants \(\alpha\)
Follow up the derivations leading to the Blasius equation leading to\[f^{\prime \prime \prime}+f f^{\prime \prime}=0\]A useful routine to solve this is BVP4C in MATLAB. Solve the Blasius equation
The following system of differential equations arises in the modeling of a stirred tank reactor with an autocatalyic reaction:\[\begin{aligned}& \frac{d x}{d t}=x-x y \\& \frac{d y}{d t}=-y+x
The Lorenz equation is widely used in the theory of non-linear equations and was encountered in modeling of natural convection. While working with these equations Lorenz discovered the phenomena of
Analyze the Frank-Kamenetskii problem for the three standard geometries of slab, cylinder, and sphere. You will need to discretize the operators suitably for the cylinder and sphere. Plot the
Verify the integral obtained by Rayleigh and hence show that the velocity profile needs to have an inflexion point for instability. Show that a simple shear flow is stable. Hence viscosity is needed
Derive the kinematic and dynamic conditions needed in the analysis. Set up the equations to find the constants. Now require that the determinant of the coefficient matrix should be zero to obtain a
A shear layer between two fluids. Assume the following velocity distribution between two shear layers:\[v_{x}(y)=v_{0} \tanh (y / \delta)\]This is known as the Betchov and Criminale form. Calculate
Stability of flow in torsional flow. Taylor determined the critical speed of rotation for flow between concentric cylinders with the inner cylinder rotating. The transition is characterized by a
Would the critical Rayleigh number for flow transition for the Bénard problem increase or decrease with the Prandtl number? Explain why in terms of the physics of the problem.
Stability analysis with heat transfer. Set up the equations for steady state for the Bénard problem.Now perturb the temperature and use an energy equation to derive an equation for the temperature
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