Thermodynamics Exam Help: Sample Questions and Tips for Success

[easylu]

Hi all, i had to do thermodynamics this year as an optional module its 100%exam and I've been trying really hard to do the exam papers before the exam in 2 weeks. I'm really confused and can't find good textbooks. can anyone help me with these sample exam questions from last year. I've been able to answer part of question 1 and 3..but nt really sure if I am right..pls help. asap.

Q1a: A radiator plate of 0.6-m × 1.0-m is allocated in a room of 20ºC. One side of the
plate is maintained at a temperature of 75ºC, the other side is assumed to be insulated,
as shown in the below figure. If the following assumptions can be made: 1) steady
operating conditions exist; 2) air is an ideal gas; 3) the local atmospheric pressure is
1.00 atm. Determine the rate of heat transfer from the plate by natural convection if the
plate is A) or D) vertical, B) horizontal with hot surface facing up; C) horizontal with hot
surface facing down.

Q1b: If the radiator surface is replaced by an extended rectangular fin surface of fin
efficiency of 0.95 and the total heat transfer surface including the rectangular fins are 2.6
times of the original flat surface given in Q1a), determine
i) The fin effectiveness;
ii) Briefly discuss how useful of applying extended surface to domestic radiators will be


I'm really lost and just need guidelines on how to approach these questions...pls help

 

[NateD]

. A1a: A) or D) vertical: The rate of heat transfer from the plate by natural convection can be calculated using the equations of convective heat transfer. The equation is given by q = h x A x (Tsurf - Tinf), where h is the convective heat transfer coefficient, A is the surface area and Tsurf and Tinf are the surface and ambient temperatures respectively. B) Horizontal with hot surface facing up: The rate of heat transfer from the plate by natural convection can be calculated using the equations of convective heat transfer. The equation is given by q = h x A x (Tsurf - Tinf), where h is the convective heat transfer coefficient, A is the surface area and Tsurf and Tinf are the surface and ambient temperatures respectively. The convective heat transfer coefficient for this orientation will be higher than the vertical orientation due to the increased exposure of the hot surface to the ambient air.C) Horizontal with hot surface facing down: The rate of heat transfer from the plate by natural convection can be calculated using the equations of convective heat transfer. The equation is given by q = h x A x (Tsurf - Tinf), where h is the convective heat transfer coefficient, A is the surface area and Tsurf and Tinf are the surface and ambient temperatures respectively. The convective heat transfer coefficient for this orientation will be lower than the vertical orientation due to the reduced exposure of the hot surface to the ambient air.Q1b: i) The fin effectiveness can be calculated using the equation η = 1 - (Tfin - Tamb)/(Tsurf - Tamb), where Tfin is the temperature of the fin, Tamb is the ambient temperature, and Tsurf is the surface temperature. ii) Applying extended surface to domestic radiators can be useful in increasing the rate of heat transfer from the radiator. The increased surface area increases the rate of convection due to increased exposure to the ambient air and the fins increase the efficiency of the heat transfer by trapping more air around the surface.