Workshop #8 - Convection Introduction
READ ABOUT FLUID CONVECTION
Convective heat transfer occurs whenever there is a fluid next to a surface with a temperature difference. Your wrist is a good example. There is air movement due to buoyancy even in a room with no apparent overall air motion. In reality there is always some air motion – if a door is opened, if a person moves, if a piece of electronics is activated. These all cause unsteady and seemingly random air motion and surface convection. Even though this heat flux may not be very large, it can still be measured with the heat flux sensors in the course kits.
The effect of fluid motion on convection is incorporated in a coefficient h that is used to correlate heat flux with the temperature difference between the surface Ts and the fluid Tfluid . For example
This convection heat transfer coefficient is non-dimensionalized in the Nusselt number by a characteristic length Lc and the thermal conductivity of the fluid k
The characteristic length is the same as used in the Reynolds number
where the fluid velocity is V and the kinematic viscosity of the fluid is ν. Correlations for the Nusselt number are often given in terms of the Reynolds number of the flow. Because of such correlations, heat transfer measurements are sometimes used as a method to infer the fluid the velocity. This will be demonstrated in the current workshop.
Your first challenge is to measure heat convection coefficients using the heat flux sensor system. Tape the heat flux sensor to the inside of your wrist to measure the surface temperature and heat flux. Keep the second thermocouple out in the room to record the air temperature. Take about 20 seconds of data under each of three conditions (it can all be recorded in the same file). First turn your wrist with the sensor facing up and not moving, then turn your wrist so the sensor is facing down not moving, then move your arm in a circle to give motion relative to the air. What is the maximum heat flux that you can achieve moving your arm?
Calculate the heat transfer coefficients and corresponding Nusselt numbers for each condition. Neglect the effects of radiation. Use the diameter of your wrist as the characteristic length, LC = d = for Re and Nu, assuming your arm is a cylinder. Neglect the effects of radiation. Record your average values for the three conditions in the table below and calculate the corresponding heat transfer coefficient and Nusselt number.
The heat transfer correlation for air flow over a cylinder at the stagnation point is
Use this correlation to estimate the maximum velocity you achieved moving your arm from your measured Nusselt number.
Air Properties: Diagrams, Equations and Calculations:
1. Re =
2. What was your maximum Velocity, V =
3. Which of the three orientations of the sensor on your wrist would you expect to give the highest heat flux? Why?
4. Which orientation would you expect to give the lowest heat flux?
Based on buoyancy, Why?