MEC701 Pre-Lab Questions W2015

Department of Mechanical and Industrial Engineering

MEC701 – Heat Transfer Pre-Laboratory Questions INSTRUCTIONS 1. 2.

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The purpose of these “pre-lab” questions is to encourage students to do some preparation before each lab session. A copy of the MEC701 Lab Manual and the property tables in the course textbook will be needed. The pre-lab question for each lab is due at the start of the lab session. Your pre-lab report also serves as a record of your attendance, which is mandatory. Your pre-lab report must use the unmodified Standard Lab Report Cover Page and the cover must have your original signature. All pre-lab questions are to be done individually. This is NOT group work. Any copying will trigger a formal charge of academic misconduct.

LAB #1: THERMAL CONDUCTIVITY AND CONTACT RESISTANCE A flow of liquid water cools the bottom end of the stainless steel bar shown in Figure 3 (p. 5) of the MEC701 Lab Manual. Water with a flow rate of 0.12 liters per minute, enters at Tin=12.5oC and exits at Tout=17.5cC. (a) Write a steady state energy balance (1st law) and calculate the heat flow rate through the bar. (b) If the thermal contact conductance at the interface of the aluminum and stainless steel bar is hc=12,600 W/m2K, what is the temperature difference across the interface (TA-TB)? LAB #2: NUMERICAL SOLUTION OF STEADY TWO-DIMENSIONAL HEAT CONDUCTION Consider two-dimensional steady conduction in the square section shown below. Two sides of the section are maintained at constant temperature (Tb=95oC). The other two surfaces are exposed to fluid convection (T∞=25oC, h=600 W/m2K). For the nodal grid and temperatures shown below calculate the unknown nodal temperatures, T1 and T2. Derivations are not needed. Do the calculation by hand using the equations provided in the lab manual.

DO NOT convert temperatures to Kelvin. (Heat conduction is driven by temperature differences.)

LAB #3: FORCED CONVECTION FROM A CYLINDER IN CROSS FLOW In this experiment the convective heat transfer from a cylinder is measured in a small wind tunnel. The air velocity (U∞) is measured using a Pitot tube connected to a manometer (see Figure 2 on p. 16 in the Lab Manual). Assume that the local atmospheric pressure is 100kPa, the ambient air temperature is T∞=20oC and the cylinder surface temperature is TS= 60oC. For a manometer reading of Δh=30 mm of water, calculate: (a) The air velocity (U∞) in the wind tunnel. (b) The Reynolds number based on the cylinder diameter. (Be sure to evaluate all the air properties at the film temperature Tf=(Ts+T∞)/2.) Describe the general nature flow near the cylinder (and in the wake) that is expected at this Reynolds number.

LAB #4: FREE CONVECTION In this experiment the free convective heat transfer rate is measured from a vertical flat plate. Assume that the local atmospheric pressure is 100kPa, the room air temperature is T∞=20oC the plate surface temperature is TS= 80oC. (a) Calculate the Grashof number based on the plate height for the vertical flat plate used in this experiment. Briefly describe the physical meaning of the Grashof number. (b) Based on the above result, is the free convective boundary layer laminar over the full height of the plate or is transition to turbulent flow expected? Discuss briefly.

LAB #5: CONCENTRIC TUBE HEAT EXCHANGER A concentric tube heat exchanger is tested in this experiment. Hot water flows in the centre pipe and cold water flows in the outer annulus. Consider the situation where liquid cold water enters the heat exchanger at Tcold,in=10oC and exits at Tcold,out= 30o C. For the pipe dimensions given in the lab manual: (a) Calculate the Reynolds number (based on the appropriate hydraulic diameter) in the annular flow for a cold water flow rate of 0.40 liters per second. Based on this result, do you expect the flow in the annulus to be laminar or turbulent? (b) Calculate the total heat transfer rate to the cold water stream using a steady-state energy balance (first law of thermodynamics).