Part G-4: Sample Exams 1
Cairo University M.S.: Eletronis Cooling Faulty of Engineering Final Exam (Sample 1) Mehanial Power Engineering Dept. Time allowed 2 Hours Solve as muh as you an. 1. A heat sink onstruted from 2024 aluminum alloy is used to ool a power diode dissipating 5 W. The internal thermal resistane between the diode juntion and the ase is 0.8 C/W, while the thermal ontat resistane between the ase and the heat sink is 10-5 m 2. C/W. Convetion at the fin surfae may be approximated as that orresponding to a flat plate in parallel flow. a) Assuming that all the diode power is transferred to the ambient air through the retangular fins; estimate the operating temperature of the diode. b) Explore options for reduing the diode temperature, subjet to the onstraints that the air veloity and fin length may not exeed 25 m/s and 20 mm, respetively, while the fin thikness may not be less than 0.5 mm. All other onditions, inluding the spaing between fins, remain as presribed. 2. Consider an array of vertial retangular fins, whih is lo be used lo ool an eletroni devie mounted in quiesent, atmospheri air at T = 27 C. Eah fin has L = 20 mm and H = 150 mm and operates at an approximately uniform temperature of T s = 77 C. For the optimum value of fin spaing S and a fin thikness of t = 1.5 mm, estimate the rate of heat transfer from the fins for an array of width W = 355 mm. 2
3a. What are the fators affeted on filter seletion to be mounted on eletroni hassis? 3b. What's the differene between ative and passive immersion ooling tehniques? 4. The interfae temperature of an eletroni assembly dissipating 10 W must be limited to 40 o C in a 50 o C environment. It is assumed that all of the generated heat will be removed by thermoeletris and that heat absorbed from the environment is negligible. The interfae temperature differene between the assembly and the thermoeletri an be held to 2 o C. The temperature differene between the thermoeletri and ambient an be held to 8 o C. The bismuth telluride element used has a length of 0.3 m and a ross-area of 0.01 m 2. Determine the size and performane harateristis of the thermoeletri temperature ontrol devie. Knowing that The equivalent material properties of the thermoeletri ouples is ρ e = 0.00267 Ω.m α e = 425 x 10-6 V/K k e = 0.00785 W/m.K NB: Design for maximum refrigeration apaity. 5. Determine the axial fore in the lead wire for the resistor shown in figure below, when bending of the PCB is inluded in the analysis over a temperature yling range from -50 to +90 C, whih produe total horizontal displaement expeted at the top of the wire will be 0.0003 in. (Note: All dimensions in inhes) Assuming that: E W = 16 x 10 6 Ib/in 2 (opper wire modulus elastiity) E P = 1.95 x 10 6 Ib/in 2 (PCB modulus of elastiity) 3
Cairo University M.S.: Eletronis Cooling Faulty of Engineering Final Exam (Sample 2) Mehanial Power Engineering Dept. Time allowed 2 Hours. Solve as muh as you an. 1. Consider a surfae-mount type transistor on a iruit board whose temperature is maintained at 35 C. Air at 20 C flows over the upper surfae of dimensions 4 mm by 8 mm with a onvetion oeffiient of 50 W/m 2.K.Three wire leads, eah of ross setion 1 mm by 0.25 mm and length 4 mm, ondut heat from the ase to the iruit board. The gap between the ase and the board is 0.2 mm. a) Assuming the ase is isothermal and negleting radiation; estimate the ase temperature when 150mW are dissipated by the transistor and (i) stagnant air or (ii) a ondutive paste fills the gap. The thermal ondutivities of the wire leads, air. And ondutive pastes are 25, 0.0263, and 0.12 W/m.K. respetively. b) Using the ondutive paste to fill the gap, we wish to determine the extent to whih inreased heat dissipation may be aommodated, subjet to the onstraint that the ase temperature not exeeds 40 C. Options inlude inreasing the air speed to ahieve a larger onvetion oeffiient h and/or hanging the lead wire material to one of larger thermal ondutivity. Independently onsidering leads fabriated from materials with thermal ondutivities of 200 and 400 W/m.K., ompute and plot the maximum allowable heat dissipation for variations in h over the range 50 h 250 W/m 2.K. 2. A 35 W power transistor is fitted to a duralumin plate 150 x 165 mm and 5 mm thik. The plate is finned on the other side by 15 fins spaed 9 mm apart. The fins are 2 mm thik and protrude 40 mm. In an ambiene of 40 C, how long would it take this transistor, after turning it on, to be within 5 o C from its final temperature? Neglet effet of radiation. 3a. Desribe with neat sketh the omponents of a heat pipe. 3b. Disuss briefly the types of Printed Wiring Board. 4
4. A irular transistor of 10 mm diameter is ooled by impingement of an air jet exiting a 2- mm diameter round nozzle with a veloity of 20 m/s and a temperature of 15 C. The jet exit and the exposed surfae of the transistor are separated by a distane of 10 mm. If the transistor is well insulated at all but its exposed surfae and the surfae temperature is not to exeed 85 o C, what is the transistor's maximum allowable operating power? 5. Determine the defletions and thermal stresses expeted in the lead wires and solder joints of the surfae mounted transformer shown in the following figure, when it is mounted on an aluminum omposite PCB whih experienes in plane (X and Y) thermal expansion during rapid temperature yling tests over a temperature range from -30 to +80 C, with no eletrial operation. (Note: All dimensions in inhes) Assuming that: a T = 35 x 10-6 in/in/ C (average TCE of transformer) a P = 20 x 10-6 in/in/ C (average TCE of omposite PCB) E W = 16x10 6 psi (modulus of elastiity, opper wire) 5
Cairo University M.S.: Eletronis Cooling Faulty of Engineering Final Exam (Sample 3) Mehanial Power Engineering Dept. Time allowed 2 Hours. Solve as muh as you an. 1. A square isothermal hip is of width w = 5 mm on a side and is mounted in a substrate suh that its side and bak surfaes are well insulated, while the front surfae is exposed to the flow of a oolant at T = 15 C. From reliability onsiderations, the hip temperature must not exeed T = 85 C. If the oolant is air and the orresponding onvetion oeffiient is h = 200 W/m 2.K. What is the maximum allowable hip power? If the oolant is a dieletri liquid for whih h = 3000 W/m 2.K. What is the maximum allowable power? 2. A ommon proedure for ooling a high-performane omputer hip involves joining the hip to a heat sink within whih irular mirohannels are mahined. During operation, the hip produes a uniform heal flux q at its interfae with the heat sink, while a liquid oolant (water) is routed through the hannels. Consider a square hip and heat sink, eah L x L on a side, with mirohannels of diameter D and pith S = C 1 D, where the onstant C 1 is greater than unity. Water is supplied at an inlet temperature T m,i and a total mass flow rate m (for the entire heat sink). a) Assuming that q is dispersed in the heat sink suh that a uniform heat flux q is maintained at the surfae of eah hannel; obtain expressions for the longitudinal distributions of the mean fluid, T m (x), and surfae. T s (x), temperatures in eah hannel. Assume laminar, fully developed flow throughout eah hannel, and express your results in terms of m. q. C 1, D, and/or L, as well as appropriate thermophysial properties. b) For L = 12 mm, D = 1 mm, C 1 = 2, q = 20 W/m 2, m = 0.010 kg/s, and T m,i = 290 K, ompute and plot the temperature distributions T m (x) and T s (x). ) A ommon objetive in designing suh heat sinks is to maximize q while maintaining the heat sink at an aeptable temperature. Subjet to presribed values of L = 12 mm and T m,i = 290 K and the onstraint that T s,max 50 C, explore the effet on q of variations in heat sink design and operating onditions. 6
3a. Desribe the limitation of operation with heat pipe. 3b. What are the onsiderations that must be taken in eletronis hassis design? 4. A thermoeletri ooling system is to be designed to ool a PCB through ooling a ondutive plate mounted on the bak surfae of the PCB. The thermoeletri ooler is aimed to maintain the external surfae of the plate at 40 o C, when the environment is 48 o C. Eah thermoeletri element will be ylindrial with a length of 0.125 m and a diameter of 0.1 m. The thermoeletri properties are: p n α (V/K) 170 x 10-6 -190 x 10-6 ρ (Ω.m) 0.001 0.0008 k (W/m K) 0.02 0.02 Assume the old juntion at 38 o C and the warm juntion at 52 o C, and the eletrial resistane of the leads and juntions = 10 % of the element resistane and design for maximum refrigeration apaity. If 10 W are being dissipated through the plate and steady-state onditions then determine: 1- Number of ouples required. 2- Rate of heat rejetion to the ambient. 3- The COP. 4- The voltage drop aross the d.. power soure. 7
5. Determine the resonant frequeny of a retangular plug-in epoxy fiberglass PCB simply supported (or hinged) on all four sides, 0.080 in thik, with a total weight of 1.0 pounds, as shown in figure. (Note: All dimensions in inhes) Assuming that: E = 2 x 10 6 Ib/in 2 (PCB modulus of elastiity) µ = 0.12 (Poisson's ratio, dimensionless) 8