双语传热学试卷1.pdf

Convective Heat Transfer Final Exam A 卷卷 2003 12 24 Note Two hour closed book final examination One crib sheet A4 both sides is permitted Calculator is allowed There are 6 problems and 100 points on the exam Point values for each question and part are listed State clearly and justify all assumptions made and label all sketches and thermal circuits clearly Your answers must be correct to 3 significant figures and have the appropriate units Allocate your time wisely and do not spend disproportionate time on any single problem Take a deep breath and relax Good luck 1 20 points A flat plate of width w 1 m and length L 1 m is maintained at a uniform surface temperature The plate is exposed to a fluid stream at 10 m s and 300 K and it has been found that the total heat transfer rate from the plate top surface only is 1000 W kinematic viscosity s T m u T KWk 01 0 thermal diffusivity sm 1 2 10 5 a What is the surface temperature Ts b Experiments show that the temperature profile along the direction normal to the surface at the trailing edge x 1m can be approximated as cybayT exp where c 1876 m 1 Estimate or explain how to calculate the magnitude of constant b K 2 20 points Two identical circular pipes of diameter 0 1 m and length 10 m are connected in series for heating a liquid from 300 K total length 20 m While the first pipe has a constant surface temperature of 400 K the second one provides a uniform heat flux of 5000 W m2 from its surface The mass flow rate is 0 5 kg s specific heat C m KkgJ p 1000 sm 24 thermal conductivity k 1W density In this problem you may assume that the flow is hydrodynamically and thermally fully developed Km 3 500mkg 10 Ts const q s const Pipe 1 Pipe 2Tm1Tm2 Tmi 300 K a Estimate the bulk fluid temperature at the end of the first pipe Tm1 i e x 10 m b Estimate the bulk fluid temperature Tm2 and surface temperature Ts2 at the end of the second pipe i e x 20 m 3 15 points Calculate heat transfer by natural convection from one side of vertical fire screen at 232 to room air at 23 The fire screen size is1 71 0 02 2 m 4 20 points Hot exhaust gases which enter a counter flow heat exchanger at 250 and leave at 143 are used to heat pressurized water at a flow rate of 2 5kg s from 35 to 85 The pressurized water specific heat is approximately 4174 KkgJ and the overall heat transfer coefficient is U 190W 2 Km a Calculate the log mean temperature difference for this counter flow heat exchanger b Determine the required surface area A using the NTU method Effectiveness of a counterflow heat exchanger 5 25 points Consider flow in a circular tube of diameter 0 1 m and length 4 m The tube surface is maintained at a constant temperature of 200 oC The fluid temperatures at the inlet and at the midpoint i e 2 m from the inlet are 50 oC and 100 oC respectively The fluid properties are as follows thermal conductivity k 0 2 W m K density 1 kg m3 specific heat Cp 1000 J kg K viscosity 2 10 4 N s m2 Assume that the flow is thermally and hydrodynamically fully developed Note You must justify the use of any convection correlation by showing that the equation is valid under the given conditions a Determine the convection heat transfer coefficient and mass flow rate 10 b What is the heat