TNix

Ok I solved the last one past exam question by myself but this one has been bugging me for days, any help would be appreciated. A composit wall consists of three materials a, b and c, two of which are of known thermal conductivity ka=20W/mK and kc=50W/mK and known thickness La=0.3m and Lc=0.15m. The third material which is sandwiched inbetween a and c is of known thickness Lb=0.15m but unknown thermal conductivity, kb. Under steady state conditions, Ts,o(temp on surface of material c)=20°C and Ts,i(temp on surface of material a)=600°C. The temp inside T∞=800°C. The inside convection coefficient h=25W/m^2K a What is the value of kb b What are the temperatures at the interfaces of a & b and b & c I thought about using the thermal resistance equations to solve it but i cant seem to get arrange an equation for it. Cheers TNix Merged post follows: Consecutive posts mergedOk I have again figured out my own answer. It was invloving the resistance equations to get q''(flux)=(T2-T1)/((La/ka)+(Lb/kb)+(Lc/kc)) then using the flux equation q''=h(deltaT) then rearranging the first to get kb. The final answer is 1.53W/mK

I found the answer, convection coefficient is just the heat transfer coefficient for convection strangly enough. All I had to do was use the equation q=m*cp*deltaT for the oil then using the same equation solve for the unknown final temp of the water. Then finding the log mean temperature and substituting it into q=U*A*deltaT and rearranging for U. Thanks anyway.

I just need some help with this past exam question. A counterflow concentric tube heat exchanger used for engine cooling in a very hot enviroment has been in servive for an extended period of time. The heat transfer surface area of the exchanger is 5m2 and the design value of the overall convection coefficient is 33W/m2k. During a test run, engine oil flowing at 0.1kg/s is cooloed from 110°C to 66°C by water supplied at a temperature of 25°C and a flow rate of 0.2kg/s. a. Calculate the operating convection coefficient b. If the ambient temperature is 50°C give a possible reason for the discrepancy