This laboratory deals with forced convection, forced convection can be considered as a staple of heat transfer. That is to say that forced convection can be found in almost any heat transfer problem, and thus understanding its importance and how it affects a given problem is one of the more important learning objectives/outcomes of heat transfer.
When dealing with forced convection the most important section, after understanding how convection works, is the convection heat transfer coefficient. The heat transfer coefficient for convection is denoted by (h) and is measured in w/m^2*K, this lab delves into the application of convection heat transfer and how it correlates to temperature, velocity, ect of the fluid in question.
The objectives for this laboratory include; determining the convective heat transfer coefficient and friction factor of the air flowing through a copper pipe, as well as evaluation of the Reynolds analogy and taking measurements of the radial velocity and temp profile in an internal pipe flow.
III. Procedure and Apparatus
A fan forces air through a long pipe with an orifice plate along the way. Before the test section there is a reduction in diametrical area which will cause an increase in velocity and a decrease in pressure. It should also be noted that the test section has a coiled heater around it which travels the length and has proper insulation.
There are seven thermocouples placed along the test section as shown in Fig 1. The exit section of the test pipe has a radial temperature and pressure measuring device as shown in figure 2. Pressures and temperatures are measured along the test section with Pitot tubes (pressure measurement) and thermocouples (temp measurement). The measured values are output on digital displays and the desired temperature is chosen by using a selection dial switch.
Orifice plate diameter 1.625"
Pipe Internal Diameter 1.249"
Heating Element Length 72"
Thermocouple Output 0.232 mV/ºF
Note – cold junctions are located at air temperature thermometer
The fan was started and the heater turned on, then the voltage control was adjusted to a maximum of 4 amperes at least an hour before the lab (This was done by the lab TA).
The experiment consists of two parts, Axial profile and Radial profile measurements;
Values of the set of parameters given in Table 1 are read from the respective displays and the procedure is repeated for 3 more sets after 5 minutes each. The 7 temperatures in Table 1, (T1 to T7) are the surface or the wall temperatures of the test section.
- Radial profile
The set of parameters given in Table 2 are read just once. The temperature measured in Table 2, (Te,1 to Te,13) are air temperatures. The apparatus should be allowed to run for 30 minutes to allow the test conditions to become
IV. Calculation method
Axial profile Calculations
Volumetric flow rate of air at orifice plate, Qo=AoCd2∆Porificeρair m3/s Where:
Cd = 0.63 discharge coefficient
Ao=πdo24 area of orifice plate (m^2)
∆Porifice pressure drop at orifice
ρair density of air
Ao=πdo24 area of the orifce (m2)
Mass flow rate of air at oriface plate, mo=ρairQo (kgs)
Inside area of test section, Ain=πdin24m2
Mass flow rate of air in test section, mtest=ρairAinVtest=ρairAin2∆Porificeρair kg/s Formula 5
Heat input as a fraction of total Heat input, Pi=Pow*1-loss*L1+L2+…+LiLtest Formula 6
Mean temp rise of fluid above entrance,∆Tmean= Pimcp℃
Heat flux, q''=PiAl= PiπdinL1-i= Piπdin(L1+L2+…+Li) W/m2 Formula 8
Convective heat transfer coefficent, hi=q''∆T=q''Ti-Ti.air Formula 9