The aim of this laboratory experiment is to analyse the rate of heat transfer losses through convection and radiation separately from a cylindrical glass rod suspended in a pressure vessel. The variation of the dimensionless quantities; Nusselt number, Prandtl number and Grashof Number, with Pressure are to be analysed graphically as well. The experimental graph of log10(Nu) vs log10(Gr.Pr) should be plotted and analysed against the graph of the empirical equations. 2. Introduction:
2.1 Theoretical Background Information:
Heat can be transferred via three different methods: conduction in solids, convection of fluids (liquid or gases) and radiation (IPAC Caltec ). The method used to transfer heat is usually the one that is the most efficient. If there is a temperature difference in a system, heat will always move down the temperature gradient (i.e. from higher to lower temperatures).
Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. (IPAC Caltec )
Convection occurs when warmer areas of a liquid of gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continuous circulation pattern and heat is transferred to cooler areas. Both conduction and convection require matter to transfer heat. (IPAC Caltec )
Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object. Objects emit radiation when high energy electrons in a higher atomic level fall down to lower energy levels. The energy lost is emitted as light or electromagnetic radiation (IPAC Caltec ). Thermal Radiation is a form of electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels. All objects absorb and emit radiation. When the absorption of energy balances the emission of energy, the temperature of an object stays constant. If the absorption of energy is greater than the emission of energy, the temperature of an object rises. If the absorption of energy is less than the emission of energy, the temperature of an object falls. (IPAC Caltec )
Qtotal= Qcond+ Qconv+Qrad (Equation 5)
All objects at temperatures above absolute zero emit thermal radiation. However, for any particular wavelength and temperature the amount of thermal radiation emitted depends on the emissivity of the object’s surface. Emissivity is defined as the ratio of energy radiated from a material’s surface to that radiated from a black body (a perfect emitter) at the same temperature and wavelength and under the same viewing conditions. It is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter). The emissivity of a surface also depends on the nature of the surface of the material. A clean and polished metal surface will have low emissivity whereas a roughened and oxidised metal surface will have a high emissivity. (National Physics Laboratory) 2.2 Background Information pertaining to the experiment:
Heat transfer from a body to its surroundings usually takes place through two processes: free or natural convection and radiation. The thermal conductivity of a gas is generally a very low value (Air, k = 0.0262 at 1 bar, 300 K) (Wikipedia ) therefore conduction heat losses is usually negligible and so can be ignored. Therefore:
Qtotal= Qconv+Qrad (Equation 5 manipulated)
The process of convection is generally a function of...