Boca Heat Transfer CH 8-11(Internal Flow, Free Convection, Boil/cond, Heat Exchangers)
The definitions for Reynolds number for flow in a circular tube are: Select all that apply: ρumD/μ(1) ρumD/v(2) umD/μ(3) 4m˙/πDμ(4) 4m˙/πDv(5) m˙D/μ(6)
1 and 4(ρumD/μ and 4m˙/πDμ)
The plot that most closely resembles the effect of Reynolds number on the friction factor for flow in a circular pipe, where LL indicates the laminar regime and TT indicates the turbulent regime, is
1: x-axis Re, Laminar left
The generally accepted value of Reynolds number for transition from laminar to turbulent flow in a pipe is about
2500
The correct definition of the Prandtl number is Pr= A- c_pμ/k B- c_pk/μ C -k/ρc_p D - μ/α
A - c_pμ/k
Air at atmospheric temperature and pressure flows in a channel of hydraulic diameter 1 μm. The characteristics of convective heat transfer for this flow are (select all that are typically correct): A. The flow is laminar. B. The mean free path of the molecules is small compared to the channel size. C. The interactions between the gas and surface molecules must be accounted for. D. The flow is turbulent.
A The flow is laminar. and C. The interactions between the gas and surface molecules must be accounted for.
The thermally fully developed region for flow in a circular tube has the following characteristics (select all that apply): A) The thermal boundary layers have merged. B) The temperature distribution is independent of axial location. C) The wall heat flux is constant. D) It is far downstream from the entrance of the tube.
A) The thermal boundary layers have merged. AND D) It is far downstream from the entrance of the tube.
In the thermal entry region for laminar flow in a circular tube, the local heat transfer coefficient is A- Higher than that in the fully developed region B- Lower than that in the fully developed region C- The same as that in the fully developed region
A- Higher than that in the fully developed region
In the thermal entry region for turbulent flow in a circular tube, the local heat transfer coefficient is A- Higher than that in the fully developed region B- Lower than that in the fully developed region C - The same as that in the fully developed region
A- Higher than that in the fully developed region
Two large parallel plates have air between them. One plate has a high temperature (T_hot) and the other a low temperature (T_Cold). The plate geometry that has a stable temperature gradient is A- Hot top cold bottom B- Cold Top hot bottom C- Hot left cold right D - Angled
A- Hot top cold bottom
The temperature distribution at a given position for flow between two parallel plates is shown. The heat transfer is not necessarily in the direction of the arrows.(RIGHT FACING PARABOLA) From this position, the fluid mean temperature A- Increases in the flow direction B- Decreases in the flow direction C- Remains the same in the flow direction
A- Increases in the flow direction
A small cold cylinder surrounded by a hot fluid is shown, with the boundary layer indicated by dashed lines. The figure that best represents the free convection velocity distribution in the fluid is A- Negative bell curve B- Negative parabola C- Positive bell curve D- Positive parabola
A- Negative bell curve
The temperature distribution at a given position for flow between two parallel plates is shown. The heat transfer is not necessarily in the direction of the arrows.(NEGATIVE PARABOLA) At this position, the heat flux at the lower plate q_0″ is A- Positive (into the fluid) B- Negative (out of the fluid) C- Zero
A- Positive (into the fluid)
he mechanisms necessary to create a fluid flow over a surface in free convection are A. gravity. B. temperature differences. C. density differences.
A. Gravity and C. Density Differences
The assumptions that are necessary to obtain a similarity solution for free convection on a vertical flat plate are A. a large Grashof-Prandtl number. B. laminar flow. C. a steady free-stream velocity.
A. a large Grashof-Prandtl number. AND B. laminar flow.
Comparing film and dropwise condensation for the same conditions, A. dropwise condensation has higher heat transfer rates. B. film condensation has higher heat transfer rates. C. film and dropwise have essentially the same heat transfer rates.
A. dropwise condensation has higher heat transfer rates.
In free convection boiling A. fluid motion is determined primarily by buoyancy. B bubbles do not form in the fluid. C. the surface temperature is above the fluid temperature and below the saturation temperature.
A. fluid motion is determined primarily by buoyancy.
For a surface of characteristic length L, the relative importance of forced and free convection depends on the ratio of Grashof and Reynolds numbers. If the ratio (Gr_L/Re_L^2) is MUCH GREATER than unity A. forced convection is negligible. B. free convection is negligible. C. forced and free convection mechanisms are comparable.
A. forced convection is negligible.
A counterflow heat exchanger is operating with heat capacity rates Ch and Cc and an overall heat transfer coefficient UAUA. If the hot fluid mass flow rate increases and the overall coefficient UA remains the same, the log mean temperature difference A. increases. B. decreases. C. remains the same.
A. increases.
If only the overall heat transfer coefficient UA of an exchanger increases, then the effectiveness A. increases. B. decreases. C. remains the same.
A. increases.
In nucleate boiling A. rougher surfaces generally give higher heat fluxes because they provide more nucleation sites. B. smoother surfaces generally give higher heat fluxes because they allow a better distribution of nucleation sites. C. the roughness of the surface has no effect.
A. rougher surfaces generally give higher heat fluxes because they provide more nucleation sites.
The fins used on horizontal tubes for condensing heat transfer are A. smaller than those for single-phase heat transfer. B. larger than those for single-phase heat transfer. C. essentially the same as those for single-phase heat transfer.
A. smaller than those for single-phase heat transfer.
In boiling and condensation, one of the dimensionless parameters that governs the heat transfer is ρg(ρl−ρv)L3/μ2. Physically, this parameter represents A. the effect of buoyancy-induced motion on heat transfer. B. the relative importance of sensible energy absorbed by the liquid to the latent energy absorbed by the vapor. C. the ratio of momentum and thermal diffusivities. D. the relative importance of buoyancy force to surface tension force.
A. the effect of buoyancy-induced motion on heat transfer.
A counterflow heat exchanger operates with the flow rates and temperatures as shown. The maximum amount of heat transfer would occur if A. the hot fluid were cooled to the cold fluid inlet temperature. B. the cold fluid were heated to the hot fluid inlet temperature. C. the hot fluid were cooled to the cold fluid inlet temperature, and the cold fluid were heated to the hot fluid inlet temperature.
A. the hot fluid were cooled to the cold fluid inlet temperature.
In boiling, the definition of excess temperature is the difference between A. the surface and saturation temperatures. B. the surface and liquid temperatures. C. the liquid and saturation temperatures.
A. the surface and saturation temperatures.
The log mean temperature difference is used to determine the heat transfer rate in a heat exchanger because A. the temperature difference between the hot and cold fluids varies with position. B. the flow rates of the two fluids are different. C. the heat transfer coefficients for the hot and cold fluid vary with position.
A. the temperature difference between the hot and cold fluids varies with position.
The effective ways to increase heat transfer rates in internal flow are? (Select all that apply) A. Making the flow passages larger B. Increasing the turbulence levels C. Increasing the surface area
All of the above
The statements that are correct about transition boiling are(select all that apply) A. the boiling oscillates between film and nucleate boiling. B. the heat flux decreases as the excess temperature is increased. C. the heat flux is less than the critical heat flux.
All of the above
An energy balance expression for flow in a tube is qconv=m˙cp(Tm,o−Tm,i). The assumptions needed to use this equation are A- There is a single inlet and a single outlet B- Steady state C- Constant fluid properties
All of the above(A,B,C)
The curve that most closely represents the variation of the local heat transfer coefficient with distance for flow in a circular tube, where x_fd,t is the thermal entry length, is
Asymptotal with high initial value of h
The figure that most accurately represents the fluid temperatures for a counterflow exchanger with Cc/Ch=0 is See answer card for answer choices
B
The correct statement about the effect of a decrease in the Reynolds number for air flow in a circular tube is: A) If the flow is laminar, it cannot become turbulent unless the wall is rough. B) If the flow is turbulent, it could become laminar. C) If the flow is laminar, it could become turbulent. D) If the flow is turbulent, it remains turbulent.
B) If the flow is turbulent, it could become laminar.
Flow patterns for free convection from a horizontal plate are shown. The flow patterns for a cold plate and a hot fluid are A. 1 and 3. B. 1 and 4. C. 2 and 3. D. 2 and 4.
B. 1 and 4.
For LAMINAR free convection flow over a vertical flat plate, the Nusselt number can be correlated with the Rayleigh number If the length of the plate is increased, the average heat transfer coefficient A- Increases B- Decreases C- Stay the same
B- Decreases
The temperature distribution at a given position for flow between two parallel plates is shown. The heat transfer is not necessarily in the direction of the arrows.(POSITIVE PARABOLA) At this position, the heat flux at the lower plate q_0″ is A- Positive (into the fluid) B- Negative (out of the fluid) C- Zero
B- Negative (out of the fluid)
An energy differential equation for laminar flow in a circular tube is u∂T/∂x = α/r*∂(r∂T∂r)/∂r. The physical significance of the LHS and RHS are: A -Term 1 represents net conduction in the x direction, and term 2 represents net conduction in the rr direction. B- Term 1 represents net energy carried by the flow (advection) in the x direction, and term 2 represents net conduction in the rr direction. C- Term 1 represents net energy carried by the flow (advection) in the x direction, and term 2 represents net advection in the rr direction. D- Term 1 represents net conduction in the x direction, and term 2 represents net energy carried by the flow (advection) in the rr direction.
B- Term 1 represents net energy carried by the flow (advection) in the x direction, and term 2 represents net conduction in the rr direction.
For a TURBULENT flow of air in a circular tube where the inlet temperature is lower than the constant surface temperature of the tube, if the mass flow rate were INCREASED , then A- Both the outlet temperature and the heat transfer rate would increase B- The outlet temperature would decrease, and the heat transfer rate would increase C- Both the outlet temperature and the heat transfer rate would decrease D- The outlet temperature would increase, and the heat transfer rate would decrease
B- The outlet temperature would decrease, and the heat transfer rate would increase
For steady flow of fluid in a circular tube, the mean temperature represents A -The average temperature of the fluid across the flow B- The thermal energy of the flow C- The centerline temperature of the fluid at any one position D- The average of the wall and the centerline temperatures
B- The thermal energy of the flow
One method of enhancing heat transfer in internal flow is to insert a coil spring inside the tube as shown. The major effect of inserting a coil spring is that A-The fluid velocities are higher due to the reduced flow area B- There is increased turbulence due to the roughness created by the spring C- There is increased heat transfer area
B- There is increased turbulence due to the roughness created by the spring
If the hot fluid in a heat exchanger is changing phase (condensing), then it behaves as if A. Ch=0 B. Ch=∞ C. ∆Tlm=0 D. ∆T1=∆T2
B. Ch=∞
A pool boiling curve for saturated water is shown. NUCLEATE boiling occurs in A. Regime 1. B. Regime 2. C. Regime 3. D. Regime 4.
B. Regime 2.
The picture that most accurately represents the cross-sectional view of condensation in a horizontal tube at low velocities is A. Vapor evenly surrounded by concentrate B. Vapor unevenly surrounded by concentrate C. Vapor on top half concentrate on bottom half D. Concentrate unevenly surrounded by vapor
B. Vapor unevenly surrounded by concentrate
Fluid temperatures as a function of position in a heat exchanger are shown.(Concentric parabolas) The flow arrangement for the heat exchanger is A. parallel-flow. B. counterflow. C. multipass.
B. counterflow.
The correct definition of the Rayleigh number for a plate of length LL is A. ρuL/μ B. gβ∆TL^3/(vα) C. hL/k D. gβ∆TL^3/v^2
B. gβ∆TL^3/(vα)
A heat exchanger operates with performance as given by the black circle. If the minimum heat capacity rate Cmin is doubled so that the NTU is decreased from 3 to 1.5, then the heat transfer rate will A. increase by about two times. B. increase by about 30%. C. remain the same. decrease by about 30%.
B. increase by about 30%.
In general, the heat transfer coefficient for turbulent film condensation A. decreases with increasing film thickness. B. increases with increasing film thickness. C. is independent of film thickness.
B. increases with increasing film thickness.
A counterflow heat exchanger is shown. The log mean temperature difference is given by ∆Tm=∆T1−∆T2/ln(∆T1/∆T2) The temperature differences ∆T1 and ∆T2 are Ta and Td are in, Tb and Tc are out. A. ∆T1=(Ta−Tb) and ∆T2=(Tc−Td). B. ∆T1=(Ta−Tc) and ∆T2=(Tb−Td) C. ∆T1=(Ta−Td) and ∆T2=(Tc−Tb).
B. ∆T1=(Ta−Tc) and ∆T2=(Tb−Td)
For LAMINAR flow in a circular tube, the figure that most correctly represents the effect of tube length on the average Nusselt number is A - Low start sideways parabola B - Low start asymptotic parabola C - High start asymptotic parabola D - Linear
C - High start asymptotic parabola
For TURBULENT flow in a circular tube, the figure that most correctly represents the effect of tube length on the average Nusselt number is A - Low start sideways parabola B - Low start asymptotic parabola C - High start asymptotic parabola D - Linear
C - High start asymptotic parabola See card with similar question for photo of graphs
The x-momentum equation for the boundary layer in free convection is given below. The different terms in the equation represent: A- 1 and 2: net advection, 3 and 4: body forces, and 5: net viscous force. B- 1 and 2: advection, 3 and 4: pressure forces, and 5: viscous force. C- 1 and 2: advection, 3: pressure force, 4: body force, and 5: viscous force. D- 1 and 2: convection, 3: pressure force, 4: body force, and 5: viscous force.
C- 1 and 2: advection, 3: pressure force, 4: body force, and 5: viscous force.
The use of the hydraulic diameter in heat transfer correlations (select all that are correct) A- Allows the laminar flow correlations for circular tubes to be used for noncircular tubes. B- Is valid only for noncircular tubes. C- Allows the turbulent flow correlations for circular tubes to be used for noncircular tubes. D- Allows correlations for turbulent flow to be used for laminar flow.
C- Allows the turbulent flow correlations for circular tubes to be used for noncircular tubes.
For fully developed turbulent flow in a circular tube, the Nusselt number can be correlated with the Reynolds number as Nu_D=C*Re_D^(0.8) where C contains the Prandtl number. This means that the heat transfer coefficient A- Is the same for tubes of different diameter B- Is larger for tubes of larger diameter C- Is smaller for tubes of larger diameter
C- Is smaller for tubes of larger diameter
The temperature distribution at a given position for flow between two parallel plates is shown. The heat transfer is not necessarily in the direction of the arrows.(NEGATIVE LINEAR) From this position, the fluid mean temperature A- Increases in the flow direction B- Decreases in the flow direction C- Remains the same in the flow direction
C- Remains the same in the flow direction
For fully developed laminar flow in a circular tube with a constant surface temperature, the Nusselt number is a constant. This means that the heat transfer coefficient is A- The same for tubes of different diameter B- Larger for tubes of larger diameter C- Smaller for tubes of larger diameter
C- Smaller for tubes of larger diameter
For TURBULENT free convection flow over a vertical flat plate, the Nusselt number can be correlated with the Rayleigh number If the length of the plate is increased, the average heat transfer coefficient A- Increases B- Decreases C- Stay the same
C- Stay the same
Three major types of heat exchanger are shown. These exchangers are classified as 1. Opposite direction arrows, 2: perpendicular arrows, 3: same direction arrows(See solution for image) A. 1 is counterflow, 2 is parallel-flow, and 3 is cross-flow. B. 1 is parallel-flow, 2 is cross-flow, and 3 is counterflow. C. 1 is counterflow, 2 is cross-flow, and 3 is parallel-flow. D. 1 is cross-flow, 2 is parallel-flow, and 3 is counterflow.
C. 1 is counterflow, 2 is cross-flow, and 3 is parallel-flow.
The curve that most correctly describes the empirical relationship between the Nusselt number and the Rayleigh number for free convection for many geometries, on a log-log plot is A. Negative linear B. 0 slope C. Positive linear
C. Positive linear
In free convection, the criterion used to indicate the transition from laminar to turbulent boundary layer flow is based on the A. Reynolds number. B. Prandtl number. C. Rayleigh number. D. Grashof number.
C. Rayleigh number.
The relation between the overall heat transfer coefficient-area product UAUA and the individual thermal resistances on the hot side, the cold side, and the wall is A. UA=Rh+Rw+Rc B. UA=1/Rh+1/Rw+1/Rc C. UA=1/(Rh+Rw+Rc) D. 1/UA=1/Rh+1/Rw+1/Rc
C. UA=1/(Rh+Rw+Rc)
For a surface of characteristic length L, the relative importance of forced and free convection depends on the ratio of Grashof and Reynolds numbers. If the ratio (Gr_L/Re_L^2) is ABOUT than unity A. forced convection is negligible. B. free convection is negligible. C. forced and free convection mechanisms are comparable.
C. forced and free convection mechanisms are comparable.
An annular enclosure is filled with air as shown. The inner cylinder is cold, and the outer cylinder is hot. The figure that most correctly represents the flow pattern inside the enclosure is A. left and right side of the cylinder flow arrows against gravity B. Top and bottom of the cylinder C. left and right side of the cylinder flow arrows with gravity D. flow Arrows following perimeter of the cylinder
C. left and right side of the cylinder flow arrows with gravity
For water vapor condensing on a flat plate, the figure that most accurately represents the velocity distribution in the liquid and vapor is A. Negative log B. negative parabola C. negative one sided bell curve D. negative linear
C. negative one sided bell curve
Heat transfer rates for dropwise condensation are higher than those for film condensation because in dropwise condensation A. the surface is covered with a small layer of liquid that has a small thermal resistance. B. the flow inside the drops is turbulent, and the mixing lowers the thermal resistance. C. the drops are small and flow off the surface, leaving bare surface with a small thermal resistance.
C. the drops are small and flow off the surface, leaving bare surface with a small thermal resistance.
Boiling occurs from a surface immersed in a liquid when A. the fluid temperature is greater than the surface temperature. B. the surface temperature is greater than the fluid temperature. C. the surface temperature is greater than the saturation temperature. D. the fluid temperature is greater than the saturation temperature.
C. the surface temperature is greater than the saturation temperature.
A counterflow heat exchanger operates with the flow rates and temperatures as shown.(T_hi>T_co&T_ho>Tci) For this situation Cmin=Cc and qmax=Cc(Tc,o−Tc,i). Cmin=Ch and qmax=Ch(Th,i−Th,o). Cmin=Cc and qmax=Cc(Th,i−Tc,i). Cmin=Ch and qmax=Ch(Th,i−Tc,i).
Cmin=Ch and qmax=Ch(Th,i−Tc,i).
The figure that most accurately represents the fluid temperatures for a counterflow exchanger with Cc = 3Ch is See answer for graph
D
The volumetric thermal expansion coefficient is a measure of the change in A- volume with temperature. B- density with pressure. C- volume with pressure. D- density with temperature.
D- density with temperature.
For a LAMINAR flow of air in a circular tube where the inlet temperature is lower than the constant surface temperature of the tube, if the mass flow rate were decreased, then A- Both the outlet temperature and the heat transfer rate would increase B- The outlet temperature would decrease, and the heat transfer rate would increase C- Both the outlet temperature and the heat transfer rate would decrease D- The outlet temperature would increase, and the heat transfer rate would decrease
D- The outlet temperature would increase, and the heat transfer rate would decrease
For turbulent flow of air in a circular tube where the inlet temperature is lower than the constant surface temperature of the tube, if the mass flow rate were For turbulent flow of air in a circular tube where the inlet temperature is lower than the constant surface temperature of the tube, if the mass flow rate were DECREASED , then A- Both the outlet temperature and the heat transfer rate would increase B- The outlet temperature would decrease, and the heat transfer rate would increase C- Both the outlet temperature, and the heat transfer rate would decrease D- The outlet temperature would increase, and the heat transfer rate would decrease
D- The outlet temperature would increase, and the heat transfer rate would decrease
A free convection hydrodynamic boundary layer on a cold vertical flat plate is surrounded by a warm fluid as shown. The figure that best represents the velocity distribution is
D- Negative bell curve looking thing srry didnt know how to describe this
The temperature difference between the hot and cold fluids is constant throughout an exchanger if the exchanger is A. a parallel-flow exchanger. B. a counterflow exchanger. C. a parallel-flow exchanger and Ch=Cc. D. a counterflow exchanger and Ch=Cc.
D. a counterflow exchanger and Ch=Cc.
A heat exchanger operates with the performance given by the black circle. If the value of Cmin is doubled while UA remains constant, then the value of effectiveness A. increases to about 0.85. B. remains roughly the same at about 0.7. C. drops to about 0.6. D. drops to about 0.45.
D. drops to about 0.45.
Which is the correct definition of the Grashof number for a plate of length LL? A. ρuL/μ B. hL/k C. u/√(gL) D. gβ∆TL^3/v^2
D. gβ∆TL^3/v^2
Water flowing in a hot tube undergoes forced convection boiling. The lowest heat transfer coefficient in the tube occurs A. near the entrance where the water is liquid. B. in the section where bubbles are beginning to form. C. in the section where there is a liquid film near the wall and vapor in the center. D. in the section where all of the water is vapor.
D. in the section where all of the water is vapor.
In boiling, one of the dimensionless parameters that governs the heat transfer is the Bond number g(ρl−ρv)L2/σ. Physically, this parameter represents A. the effect of buoyancy-induced motion on heat transfer. B. the relative importance of sensible energy absorbed by the liquid to the latent energy absorbed by the vapor. C. the ratio of momentum and thermal diffusivities. D. the relative importance of buoyancy force to surface tension force.
D. the relative importance of buoyancy force to surface tension force.
Water enters a circular tube at a mean temperature of Tm,i. The tube is electrically heated so that there is a uniform heat flux into the fluid. The curve that most accurately describes the variation of the mean temperature Tm as a function of x is
Linear
A pool boiling curve for saturated water is shown. FILM boiling occurs in A. Regime 1. B. Regime 2. C. Regime 3. D. Regime 4.
Regime 4.
A fluid enters a circular tube at a mean temperature that is lower than the tube surface temperature. The curve that most accurately resembles the temperature as a function of radius past the thermal entry length is
Right facing semi-circle