HEAT TRANSFER MECHANISM
Heat Transfer by Conduction
Heat Transfer by Radiation
Heat transfer by Convection
Major factor is the vibrational energy and cloud of electron which is free to move within the lattice structure.
Metal much better better conductor than non-metal.
Principle mechanism involved in heat transfer process of solid material.
Thermal Conductivity = To quantify ability of substance to conduct heat.
In steady state, rate of heat transfer depend on cross sectional area of surface, temperature gradient and thermal conductivity of material.
Heat Transfer Process
Steady-state = Temperature remain constants at any point over time while heat flowing. Eg: heating and cooling through heat exchanger and holding tubes.
Unsteady state = Temperature at any point change over time while heat is flowing.
In porous solid (food), thermal conductivity depend on composition and other factor (void spaces, fluid contained in pores and homogeneity.
In fibrous structure food (meat), thermal conductivity cannot be the same in different direction. Heat flow path change with different direction.
Thermal Diffusivity = Ratio of the thermal conductivity to the specific heat of the product multiplied by its density.
In physical terms, gives measure of how quickly the temperature will change when it is heated or cooled.
Materials with a high thermal diffusivity will heat or cool quickly & substances with a low thermal diffusivity will heat or cool slowly.
Heat transfer occurs when heat energy is carried along by a moving fluid in contact with a solid surface.
Natural Convection = natural buoyancy effect of decreased density with increased temperature
Forced convection = artificially by imparting mechanical energy to the fluid, such as with pumps or blowers for liquids and gases, respectively
Complicated to determine because of the fluid motion.
When fluid flowing, shear stress occur in fluid near the surface because of viscous properties of fluid.
Molecules at the surface try to attach themselves to the surface while neighboring molecules are trying to pass them by within the bulk fluid flow
Cause a velocity profile to develop near the surface.
Fluid next to the surface does not move but sticks to it, while neighboring fluid flow is slowed down by the friction of trying to pass the stationery molecules.
Fluid velocity gradually increases with distance away from the surface until the region of bulk fluid flow is reached, where the fluid velocity is all the same at the maximum.
This region is known as a boundary layer.
Newton's Law of Heat Convection = Rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings
Heat Transfer Coefficient = Measure of the resistance to heat flow, caused by the boundary film.
Radiation will travel through a vacuum at the speed of light, When contacting a further object, this radiation will be reflected, transmitted or absorbed.
Only radiation that is absorbed will impart its energy and cause a temperature change.
Characteristic of electromagnetic radiation
Consists of an electric and magnetic field mutually at right angles
Both fields vary sinusoidally with time and distance
The type of radiation is characterized by its wavelength λ or its frequency f.
The relationship:
frequency X wavelength = velocity of light
Infrared radiation can be used for heating purposes and for drying application.
Frying
Mainly used to alter the eating quality of a food.
When food is placed in hot oil, the surface temperature rises rapidly and water is vaporised as steam. The surface then begins to dry out.
The plane of evaporation moves inside the food, and a crust is formed
The surface temperature of the food then rises to that of the hot oil, and the internal temperature rises more slowly towards 100⁰C.
The rate of heat transfer is controlled by the temperature difference between the oil and the food and by the surface heat transfer coefficient
During frying, both water and water vapour are removed from the larger capillaries first, and replaced by hot oil.
Moisture moves from the surface of the food through a boundary film of oil.