Fundamentals of Heat and Mass Transfer
Fundamentals of Heat and Mass Transfer
7th Edition
ISBN: 9780470917855
Author: Bergman, Theodore L./
Publisher: John Wiley & Sons Inc
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Chapter 8, Problem 8.117P

An experiment is devised to measure liquid flow andconvective heat transfer rates in microscale channels.The mass flow rate through a channel is determined bymeasuring the amount of liquid that has flowedthrough the channel and dividing by the duration ofthe experiment. The mean temperature of the outletfluid is also measured. To minimize the time needed toperform the experiment (that is. to collect a significant amount of liquid so that its mass and temperature canbe accurately measured),arrays of microchannels aretypically used. Consider an array of microchannels ofcircular cross section, each with a nominal diameter of 50 μ m , fabricated into a copper block. The channels are 20 mm long, and the block is held at 310 K. Water at an inlet temperature of 300 K is forced into the channels from a pressurized plenum, so that a pressure difference of 2.5 × 10 6 Pa exists from the entrance to the exit of each channel. In many microscale systems, the characteristic dimensions are similar to the tolerances that can be controlled during the manufacture of the experimental apparatus. Hence, careful consideration of the effect of machining tolerances must be made when interpreting the experimental results.
(a) Consider the case in which three microchannels are machined in the copper block. The channel diameters exhibit some deviation due to manufacturing constraints and at of actual diameter 45 μ m , 50 μ m , and 55 μ m respectively. Calculate the mass how rate through each of the three channels, along with the mean outlet temperature of each channel.
(b) If the water exiting each of the three channels is collected and mixed in a single container, calculate the average how rate through each of the three channels and the average mixed temperature of the water that is collected from all three channels.
(c) The enthusiastic experimentalist uses the averageflow rate and the average mixed outlet temperature to analyze the performance of the average 50 μ m diameter channel and concludes thatflow rates and heat transfer coefficients areincreased and decreased. respectively, by about 5% when forced convection occurs in microchannels. Comment on the validity of the experimentalist’s conclusion.

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Chapter 8 Solutions

Fundamentals of Heat and Mass Transfer

Ch. 8 - Consider a circular tube of diameter D and length...Ch. 8 - Consider flow in a circular tube. Within the test...Ch. 8 - Consider a cylindrical nuclear fuel rod of length...Ch. 8 - Consider the laminar thermal boundary layer...Ch. 8 - In a particular application involving fluid flow...Ch. 8 - A flat-plate solar collector is used w heat...Ch. 8 - Atmospheric air enters the heated section of a...Ch. 8 - Fluid enters a tube with a flow rate of 0.015kg/s...Ch. 8 - Water at 300 K and a flow rate of 5kg/s enters a...Ch. 8 - Slug flow is an idealized tube flow condition for...Ch. 8 - Superimposing a control volume that is...Ch. 8 - An experimental nuclear core simulation apparatus...Ch. 8 - Water at 20°C and a flow rate of 0.1kg/s enters a...Ch. 8 - Engine oil is heated by flowing through a circular...Ch. 8 - Engine oil flows through a 25mm -diameter tube at...Ch. 8 - In the final stages of production, a...Ch. 8 - An oil preheater consists of a single tube of 10mm...Ch. 8 - Engine oil flows at a rate of 1kg/s through a 5mm...Ch. 8 - Air at p=1atm enters a thin-walled ( D=5-mm...Ch. 8 - To cool a summer home without using a vapor...Ch. 8 - Batch processes are often used in chemical and...Ch. 8 - The evaporator section of a heat pump is installed...Ch. 8 - Water flowing at 2kg/s through a 40mm diameter...Ch. 8 - Consider the conditions associated with the hot...Ch. 8 - A thick-walled, stainless steel (AISI 316) pipe of...Ch. 8 - An air heater for an industrial application...Ch. 8 - Consider fully developed conditions in a circular...Ch. 8 - Consider the encased pipe of Problem 4.29, but now...Ch. 8 - Water flows through a thick-wailed tube with an...Ch. 8 - Atmospheric air enters a 10m -long. 150mm...Ch. 8 - NaK (45%/55). which is an alloy of sodium and...Ch. 8 - The products of combustion from a burner are...Ch. 8 - Liquid mercury at 0.5kg/s is lo be heated from 300...Ch. 8 - The surface of a 50-mm-diameter. thin-walled tube...Ch. 8 - Consider a horizontal, thin-walled circular tube...Ch. 8 - Consider pressurized liquid water flowing at...Ch. 8 - Cooling water flows through the 25.4-mm -diameter...Ch. 8 - The air passage for cooling a gas turbine vane can...Ch. 8 - The core of a high-temperature, gas-cooled nuclear...Ch. 8 - Air at 200kPa enters a 2-m -long, thin-walled tube...Ch. 8 - Heated air required for a food-drying process is...Ch. 8 - Consider laminar flow of a fluid with Pr=4 that...Ch. 8 - A common procedure for cooling a high-performance...Ch. 8 - One way to cool chips mounted on the circuit...Ch. 8 - Refrigerant- 134a is being transported a 0.1 kg/s...Ch. 8 - Oil at 150°C flows slowly through a long,...Ch. 8 - Exhaust gases from a wire processing oven are...Ch. 8 - A hot fluid passes through a thin-walled tube of...Ch. 8 - Consider a thin-walled tube of 10mm diameter and...Ch. 8 - Water at a flow rate of m =0.215kg/s is cooled...Ch. 8 - To maintain pump power requirements per unit flow...Ch. 8 - Consider a thin-walled, metallic tube of length...Ch. 8 - A circular tube of diameter D=0.2mm and length...Ch. 8 - Repeat Problem 8.66 for a circular tube of...Ch. 8 - Heat is to be removed from a reaction vessel...Ch. 8 - A healing contractor must heat 0.2kg/s of water...Ch. 8 - A thin-walled tube with a diameter of 6 mm and...Ch. 8 - A 50mm -diameter, thin—walled metal pipe covered...Ch. 8 - A thin-walled, uninsulated 0.3m -diameter duct is...Ch. 8 - Pressurized water at Tm,i=200C is pumped at...Ch. 8 - Water at 290K and 0.2kg/s flows through a Teflon...Ch. 8 - The temperature of flue gases flowing through the...Ch. 8 - In a biomedical supplies manufacturing process, a...Ch. 8 - Consider the ground source heat pump of Problem...Ch. 8 - For a sharp-edged inlet and a combined entry...Ch. 8 - Fluid enters a thin-walled rube of 5-mni diameter...Ch. 8 - Air at 3104kg/s and 27C enters a rectangular duct...Ch. 8 - Air at 25C flows at 30106kg/s within 100mm -long...Ch. 8 - A cold plate is an active cooling device that is...Ch. 8 - The cold plate design of Problem 8.82 has not been...Ch. 8 - A device that recovers heat from high-temperature...Ch. 8 - Air at 1 atm and 285K enters a 2-m -long...Ch. 8 - A double-wall heat exchanger is used to transfer...Ch. 8 - Consider laminar, fully developed flow in a...Ch. 8 - You have been asked to perform a feasibility study...Ch. 8 - A coolant flows through a rectangular channel...Ch. 8 - An electronic circuit board dissipating 50W is...Ch. 8 - To slow down large prime movers like locomotives,...Ch. 8 - A printed circuit board (PCB) is cooled by...Ch. 8 - Water at m=0.02kg/s and Tm,i=20C enters an annular...Ch. 8 - tFor the conditions of Problem 8.93, how tong must...Ch. 8 - Referring 10 Figure 8.11, consider conditions in...Ch. 8 - Consider the air healer of Problem 8.38, but now...Ch. 8 - Consider a concentric tube annulus for which the...Ch. 8 - It is common practice (o recover waste heat from...Ch. 8 - A concentric lube arrangement, for which the inner...Ch. 8 - Consider sterilization of the pharmaceutical...Ch. 8 - An engineer proposes to insert a solid rod of...Ch. 8 - An electrical power transformer of diameter 230mm...Ch. 8 - A bayonet cooler is used to reduce the temperature...Ch. 8 - The mold used in an injection molding process...Ch. 8 - Prob. 8.107PCh. 8 - Prob. 8.108PCh. 8 - Consider the microchannel cooling arrangement...Ch. 8 - The onset of turbulence in a gas flowing within a...Ch. 8 - Due to its comparatively large thermal...Ch. 8 - A novel scheme for dissipating heat from the chips...Ch. 8 - An experiment is designed to study microscale...Ch. 8 - Determine the tube diameter that corresponds to a...Ch. 8 - An experiment is devised to measure liquid flow...Ch. 8 - In the processing of very long plastic tubes of...Ch. 8 - Air at 300K and a flow rate of 3kg/h passes upward...Ch. 8 - What is the convection mass transfer coefficient...Ch. 8 - Air flowing through a tube of 75mm diameter passes...Ch. 8 - Consider gas flow of mass density and rate m...Ch. 8 - Atmospheric air at 25C and 3104kg/s flows through...Ch. 8 - Air at 25C and 1atm is in fully developed flow at...Ch. 8 - A humidifier consists of a bundle of vertical...Ch. 8 - The final step of a manufacturing process in which...Ch. 8 - Dry air is inhaled at a rate of lo liter/win...Ch. 8 - A mass transfer Operation is preceded by laminar...
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