Heat Transfer from a Single, Heated Block

Transcription

1 Heat Transfer from a Single, Heated Block Gerald Recktenwald Portland State University Department of Mechanical Engineering gerry@me.pdx.edu May 16, 2006 These slides are a supplement to the lectures in ME 449/549 Thermal Management Measurements and are c 2006, Gerald W. Recktenwald, all rights reserved. The material is provided to enhance the learning of students in the course, and should only be used for educational purposes. The material in these slides is subject to change without notice. The PDF version of these slides may be downloaded or stored or printed only for noncommercial, educational use. The repackaging or sale of these slides in any form, without written consent of the author, is prohibited. The latest version of this PDF file, along with other supplemental material for the class, can be found at Note that the location (URL) for this web site may change. Version 0.46 May 16, 2006 page 1

2 Overview Schematic of Basic Experiment Measuring the Heat Transfer Coefficient Heater Circuit Experiment Planning Uncertainty analysis Relationship to Practical Thermal Management Problems Heat Transfer from a Block page 2 Measurement of Heat Transfer Coefficient (1) T a V Q rad Q conv Q cond Calculate the heat transfer coefficient from measured quantities h = Q conv A s ( T s T a ) (1) where T s = T a = A s = Q conv = average surface temperature of the block temperature of air approaching the block surface area of block that is participating in the convection convective heat loss from the block Heat Transfer from a Block page 3

3 Measurement of Heat Transfer Coefficient (2) Not all of the electrical power input is lost directly by convection. An energy balance on the block gives Q conv = Ėin Q cond Q rad (2) Q conv = Ė in = Q cond = Q rad = convective heat loss electrical power input conduction heat transfer from block to board radiation heat transfer to surroundings Equation (2) is a centering correction. Heat Transfer from a Block page 4 Device Mock-up Heater assembly: Exploded view with potential temperature measurement locations: Balsa Foil heater Machine screws Aluminum blocks inch thick Simulated PCB Foil heater Balsa Aluminum blocks inch thick 6 Simulated PCB Heat Transfer from a Block page 5

4 Measuring Power Input (1) V sh 110 VAC DC Power Supply + shunt I sh V h heater The power dissipated by the electric resistance heater is Ė in = V h I h (3) where V h is the voltage across the heater, and I h is the current through the heater. The shunt resistor is used to measure I h. From a measurement of V sh we compute I h = I sh = V sh R sh = Ėin = V hv sh R sh (4) Heat Transfer from a Block page 6 Measuring Power Input (2) Measuring power with a shunt resistor is much more accurate than measuring just the voltage across the heater and computing Ėin = V 2 h /R h because R h varies with temperature. The shunt resistor has a very low and stable resistance (R sh Ω). Since R sh is very small, the shunt resistor dissipates very little power. Thus its temperature will be stable and its resistance will not change during the experiment. Heat Transfer from a Block page 7

5 Measuring Power Input (3) Sample Power Calculation: Assume that R sh = Ω and that V sh = V and V h = 8.5 V were measured. Then Ė in = (8.5)( ) = 4.76 W and E shunt = V 2 shunt R sh = ( ) = W Heat Transfer from a Block page 8 Experiment Planning 1. Decide which type of sensors, and how many of each type are needed to measure all quantities used to compute h from h = Q conv A( T s T a ) 2. Develop a wiring diagram for your experiment. 3. Fabricate thermocouples and attach them to your apparatus. 4. Mount your apparatus to a bottom hatch for the wind tunnel. 5. Route the sensor leads to a terminal strip. Label all wires and document the labeling scheme in your notebook and your report. Heat Transfer from a Block page 9

6 Model of Start-up Transient (1) How long will you have to wait for steady state? U a, T a Q c = ha(t s T a ) I 2 R Use a simplified model to estimate the time it takes the block to reach its steady state temperature. Assumptions: Block has uniform temperature All heat is lost by convection Heat Transfer from a Block page 10 Model of Start-up Transient (2) A transient energy balance on the block gives mc dt dt = Ėin ha(t T a ) (5) where Ėin is the electrical power input, h is the heat transfer coefficient. Use a shifted temperature θ, and the response time τ θ = T T a to transform Equation (5) to dimensionless form τ = mc ha (6) dθ dt = Ėin mc θ τ (7) Heat Transfer from a Block page 11

7 Model of Start-up Transient (3) With the initial condition θ(t = 0) = θ i = T i T a the solution to Equation (7) is θ = Ėin ha + θ i Ėin ha! e t/τ T! T a C h = 5 W/m 2 /C h = 10 W/m 2 /C h = 20 W/m 2 /C h = 40 W/m 2 /C time (minutes) Heat Transfer from a Block page 12 Define T ss as the steady state temperature Model of Start-up Transient (4) T ss = T a + Ėin ha and t p as the time for block to reach p percent of its final temperature difference With our model we compute T T a T ss T a = p 100 h τ T ss t 50 t 90 t 99 (W/m 2 / C) (min) ( C) (min) (min) (min) Heat Transfer from a Block page 13

8 Uncertainty Analysis (1) Suppose that your data reduction equation had the form h = f(v h, V sh, R sh, A s, T a, T w, T bt, T bb, k b, ε) (8) V h = V sh = R sh = A s = T a = T w = T bt = T bb = k b = ε = voltage across the heater voltage across the shunt resistor resistance of shunt resistor surface area of the block temperature of oncoming air temperature of duct walls temperature on the top surface of the board temperature on the bottom surface of the board thermal conductivity of the board emissivity of the heated block Data analysis involves creating a routine to compute h, and perturbation of all inputs to estimate u h. Heat Transfer from a Block page 14 Uncertainty Analysis (2) Given the data reduction formula in Equation (8) u hvh = f(v h + u Vh, V sh, R sh, A s, T a, T w, T bt, T bb, k b, ε) h u hvsh = f(v h, V sh + u Vsh, R sh, A s, T a, T w, T bt, T bb, k b, ε) h where. u Vh u Vsh. u h = q (u hvh ) 2 + (u hvsh ) the total uncertainty in the heater voltage the total uncertainty in the voltage across the shunt resistor u Vh, u Vsh, etc. depend on random error, instrument error, and calibration error. Heat Transfer from a Block page 15

9 Shakedown An end-to-end calibration is also called first order replication Used to verify equipment Make measurements of known quantities Run experiment with power off: All temperature readings should be the same, i.e., room temperature Perform benchmark experiments: Can you repeat experiments and obtain the same results published by others? One outcome is an estimate of random component of fixed error Run the no-power experiment Wait for system to come into steady state Over time period of typical data collection, take 30 (or more) samples from each sensor Compute average and standard deviation of sensor values u xrandom = 2σ for 20:1 odds (95 % confidence) Heat Transfer from a Block page 16 Relationship to Practical Problems Heaters as Prototype Devices Develop experience with multimode heat transfer Gain familiarity with complexity of measurements Gain experience building a thermal mock-up References [1] R. J. Moffat. Uncertainty analysis. In K. Azar, editor, Thermal Measurements in Electronic Cooling, pages CRC Press, Boca Raton, FL, Heat Transfer from a Block page 17