London: Elsevier Science Publ. McAdams W. Heat transmission. New York: McGraw-Hill.

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Food Technology 53 3 : 72— Moreira, R. Deep fat frying: Fundamentals and applications. Mudgett, R. Developments in microwave food processing. In Biotechnology and food process engineering. Schwartzberg and M. Rao, eds. Electrical properties of foods. Muller-Steinhagen, H. Plate heat exchangers: Past-present-future. Norris, J. Optimize heat exchanger cleaning schedules. Chemical Engineering Progress 97 6 : 56— Ohlsson, T. Industrial uses of dielectric properties of foods.

In Physical properties offoods Jowitt, E. Escher, M. Kent, B. McKenna, and M. Rogues, eds.

London: Elsevier Applied Science, — Palazoglu, T. Computational fluid dynamics modeling of fluid flow in helical tubes. Journal of Food Process Engineering 25 2 — Perry, R. Rahman, S. Food properties handbook. Rao, M. Rheology of fluid and semisolid foods. Reuter, H. Aseptic processing of food.

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Lancaster, PA: Technomic Publ. Rossell, J. Improving quality. Saravacos, G. Transport properties of foods. Heating rates of fruit products in an agitated kettle. Food Technology 29 3 : 54A—58A. Schiffmann, R. Microwave and dielectric drying. In Handbook of industrial drying. Mujumdar, ed. Schormueller, J. Die Erhaltung der Lebensmittel. Stuttgart: Ferdinand Euke. Singh, R. Heating and cooling processes for foods.

In Handbook of food engineering. Heldman and D. Lund, eds. McGraw-Hill Education. Cite Close Cite this book. Eduardo Cao. Download as Close Share this book. Summary PDF. Table of Contents. Related searches Search AccessEngineering for other content tagged with these It is the transfer of energy by means of photons in electromagnetic waves governed by the same laws.

## Mass transfer

Heat is defined in physics as the transfer of thermal energy across a well-defined boundary around a thermodynamic system. The thermodynamic free energy is the amount of work that a thermodynamic system can perform. Enthalpy is a thermodynamic potential , designated by the letter "H", that is the sum of the internal energy of the system U plus the product of pressure P and volume V.

Joule is a unit to quantify energy , work, or the amount of heat. Heat transfer is a process function or path function , as opposed to functions of state ; therefore, the amount of heat transferred in a thermodynamic process that changes the state of a system depends on how that process occurs, not only the net difference between the initial and final states of the process. Thermodynamic and mechanical heat transfer is calculated with the heat transfer coefficient , the proportionality between the heat flux and the thermodynamic driving force for the flow of heat.

Heat flux is a quantitative, vectorial representation of heat-flow through a surface.

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In engineering contexts, the term heat is taken as synonymous to thermal energy. This usage has its origin in the historical interpretation of heat as a fluid caloric that can be transferred by various causes, [3] and that is also common in the language of laymen and everyday life. The transport equations for thermal energy Fourier's law , mechanical momentum Newton's law for fluids , and mass transfer Fick's laws of diffusion are similar, [4] [5] and analogies among these three transport processes have been developed to facilitate prediction of conversion from any one to the others.

Thermal engineering concerns the generation, use, conversion, and exchange of heat transfer. As such, heat transfer is involved in almost every sector of the economy. By transferring matter, energy—including thermal energy—is moved by the physical transfer of a hot or cold object from one place to another. A practical example is thermal hydraulics. On a microscopic scale, heat conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy heat to these neighboring particles.

In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from one atom to another. Conduction is the most significant means of heat transfer within a solid or between solid objects in thermal contact. Fluids—especially gases—are less conductive. Thermal contact conductance is the study of heat conduction between solid bodies in contact. Steady state conduction is an idealized model of conduction that happens when the temperature difference driving the conduction is constant, so that after a time, the spatial distribution of temperatures in the conducting object does not change any further see Fourier's law.

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Transient conduction see Heat equation occurs when the temperature within an object changes as a function of time. Analysis of transient systems is more complex, and analytic solutions of the heat equation are only valid for idealized model systems. Practical applications are generally investigated using numerical methods, approximation techniques, or empirical study. In this case the fluid is forced to flow by using a pump, fan or other mechanical means.

Convective heat transfer , or convection, is the transfer of heat from one place to another by the movement of fluids , a process that is essentially the transfer of heat via mass transfer.

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Bulk motion of fluid enhances heat transfer in many physical situations, such as for example between a solid surface and the fluid. Although sometimes discussed as a third method of heat transfer, convection is usually used to describe the combined effects of heat conduction within the fluid diffusion and heat transference by bulk fluid flow streaming. Free, or natural, convection occurs when bulk fluid motions streams and currents are caused by buoyancy forces that result from density variations due to variations of temperature in the fluid.

Forced convection is a term used when the streams and currents in the fluid are induced by external means—such as fans, stirrers, and pumps—creating an artificially induced convection current. Convective cooling is sometimes described as Newton's law of cooling :. The rate of heat loss of a body is proportional to the temperature difference between the body and its surroundings.

However, by definition, the validity of Newton's law of Cooling requires that the rate of heat loss from convection be a linear function of "proportional to" the temperature difference that drives heat transfer, and in convective cooling this is sometimes not the case.

In general, convection is not linearly dependent on temperature gradients, and in some cases is strongly nonlinear. In these cases, Newton's law does not apply. In a body of fluid that is heated from underneath its container, conduction and convection can be considered to compete for dominance. If heat conduction is too great, fluid moving down by convection is heated by conduction so fast that its downward movement will be stopped due to its buoyancy , while fluid moving up by convection is cooled by conduction so fast that its driving buoyancy will diminish.

On the other hand, if heat conduction is very low, a large temperature gradient may be formed and convection might be very strong. It is a measure which determines the relative strength of conduction and convection. The Rayleigh number can be understood as the ratio between the rate of heat transfer by convection to the rate of heat transfer by conduction; or, equivalently, the ratio between the corresponding timescales i. This can be seen as follows, where all calculations are up to numerical factors depending on the geometry of the system.