Heat Transfer

introduction• Heat transfer is the movement of energy from one point to another by

virtue of a difference in temperature. • Heating and cooling are manifestations of this phenomenon, which is

used in industrial operations and in domestic activities. • Increasing energy costs and in some cases inadequate availability of

energy will require peak efficiency in heating and cooling operations. An understanding of the mechanisms of heat transport is needed in order to recognize limitations of heating and cooling systems, which can then lead to adoption of practices that circumvent these limitations.

• In industrial and domestic heating and cooling, energy-use audits can be used to determine total energy use and the distribution within the process, to identify areas of high energy use, and to target these areas for energy conservation measures.

MECHANISMS OF HEAT TRANSFER

• Heat Transfer by Conduction• Heat Transfer by Convection• Heat Transfer by Radiation

Heat Transfer by Conduction• When heat is transferred between adjacent molecules, the process is

called conduction. • This is the mechanism of heat transfer in solids.• Fourier’s First Law of Heat Transfer : the heat flux, in conduction heat

transfer, is proportional to the temperature gradient.

• q is the rate of heat flow, and A is the area through which heat is transferred. A is the area perpendicular to the direction of heat flow. The expression q/A, the rate of heat transfer per unit area, is called the heat flux.

• The negative sign in Equation indicates that positive heat flow will occur in the direction of decreasing temperature.

Estimation of Thermal Conductivity of Food Products

• The thermal conductivity of materials varies with the composition and, in some cases, the physical orientation of components. Foods, being of biological origin possess highly variable composition and structure, therefore, k of foods presented in the tables is not always the same for all foods in the category listed.

• The effect of variations in the composition of a material on values of the thermal conductivity, has been reported by Choi and Okos (1987).

• k is calculated from the thermal conductivity of the pure component ki and the volume fraction of each component, Xvi.

• An important assumption used in this estimation procedure is that the contribution of each component to the composite thermal conductivity is proportional to the component volume fraction

• The thermal conductivity in W/(m · K) of pure water (kw), ice (kic), protein (kp), fat (kf), carbohydrate (kc), fiber (kfi), and ash (ka) are calculated at the temperature, T in ◦C,

Steady-state conduction

• Pindah panas pada permukaan datar– Single layer– multilayer

• Pindah panas pada permukaan lengkung– Single layer– multilayer

Pindah panas konveksi

• Konveksi alami• Konveksi paksa• The heat transfer coefficient convective h is a

fundamental notion in heat transfer (W/m2K), it is determined experimentally.

• The value depends on the properties of the fluid (specifi c heat, viscosity, density and thermal conductivity), the turbulence (average velocity) and the geometry of the system.

Persamaan

The exponent of Pr is 0.3 for heating and 0.4 for cooling. The dimension d is theinside diameter of the pipe. The fluid properties are taken at the average temperature,except for μ0 which is the viscosity at the temperature at the exchange surface.

Konveksi alami

Konveksi paksa turbulen

Estimate the convective heat transfer coefficient between orange juice fl owinginside a pipe and the pipe wall. The juice is being cooled.Data:Pipe diameter: 0.05 mVolumetric fl ow rate: 4 m3/h