Heat Transfer Involving Liquids

Liquids can evaporate even below their boiling points. The water in perspiration is an effective coolant for our bodies. Each gram of water that evaporates absorbs 2.41 kJ of heat from the body. We feel even cooler in a breeze because perspiration evaporates faster, so heat is removed more rapidly. 

Liquid. Molccula r 13 eight (g/mol) apor Pressure (ton- at lire.) Boiling Point at 1 atm ( Ci) IIc.it of Vapori/alion at Point J/g kJ/mol 

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Condensation is the reverse of evaporation. The amount of heat that must be removed from a vapor to condense it (without change in temperature) is called the heat of condensation. 

The heat of condensation of a liquid is equal in magnitude to the heat of vaporization. It is released by the vapor during condensation. 

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In the Taylor-Prandtl modification of the theory of heat transfer to a turbulent fluid, it was assumed that the heat passed directly from the turbulent fluid to the laminar sublayer and the existence of the buffer layer was neglected. It was therefore possible to apply the simple theory for the boundary layer in order to calculate the heat transfer. In most cases, the results so obtained are sufficiently accurate, but errors become significant when the relations are used to calculate heat transfer to liquids of high viscosities. A more accurate expression can be obtained if the temperature difference across the buffer layer is taken into account. The exact conditions in the buffer layer are difficult to define and any mathematical treatment of the problem involves a number of assumptions. However, the conditions close to the surface over which fluid is flowing can be calculated approximately using the universal velocity profile,(10)... 

In the preceding chapters we considered forced and free convection heal transfer involving a single phase of a fluid. The analysis of such convectior processes involves tlie thermophysical properties p, p, k, and Cp of the fluid The analysis of boiling heat transfer involves these properties of the liquic (indicated by the subscript 0 or vapor (indicated by the subscript v) as well a the properties (the latent heat of vaporization) and cr (the surface tension) The hf represents the energy absorbed as a unit mass of liquid vaporize ... 

In this paper we attempt to review some of the reeent studies on microscale heat transfer involving vapour and liquid two-phase flow covering both experimental and analytical aspects. Furthermore we link these studies to interpreting and understanding boiling heat transfer, and in partieular augmentation of flow boiling heat transfer. 

In many cases of heat transfer involving either a liquid or a gas, convection is an important factor. In the majority of heat-transfer cases met in industrial practice, heat is being transferred from one fluid through a solid wall to another fluid. Assume a hot fluid at a temperature ti flowing past one side of a metal wall and a cold fluid at t flowing past the other side to which a scale of thickness x, adheres. In such a case, the conditions obtaining at a given section are illustrated dia-... 

Heat is involved in most real-life processes. This permits heat—into or out of a system—to serve as a universal detector. In many cases, the heat into or out of a system can be measured nondestruc-tively. Heat transfer occurs in three ways conduction, convection, and radiation. Conduction occurs between solid materials when placed in contact with each other. Convection occurs when a hot material and a cold material are separated by a fluid (gas or liquid). Radiative heat transfer involves the emission and consequent absorption of electromagnetic radiation between a hot and cold material. 

This varies in many cases in the rangebetween 3 and 7 for straight channels involving gaseous medium at low pressure drop. Despite these relatively low numbers, the small diffusion paths allow for fest heat transfer. For liquids much higher Nu numbers are achievable owing to the higher heat conductivity of the medium. 

An important mixing operation involves bringing different molecular species together to obtain a chemical reaction. The components may be miscible liquids, immiscible liquids, solid particles and a liquid, a gas and a liquid, a gas and solid particles, or two gases. In some cases, temperature differences exist between an equipment surface and the bulk fluid, or between the suspended particles and the continuous phase fluid. The same mechanisms that enhance mass transfer by reducing the film thickness are used to promote heat transfer by increasing the temperature gradient in the film. These mechanisms are bulk flow, eddy diffusion, and molecular diffusion. The performance of equipment in which heat transfer occurs is expressed in terms of forced convective heat transfer coefficients. 

The processing industry has given operations involving heat transfer to a boiling liquid the general name of evaporation. The most common application is the removal of water from a processing stream. Evaporation is used in the food, chemical, and petrochemical industries, and usually it results in an increase in the concentration of a certain species. 

Liquid Pool Flames. Liquid fuel or flammable spills often lead to fires involving a flame at the surface of the liquid. This type of diffusion flame moves across the surface of the liquid driven by evaporation of the fuel through heat transfer ahead of the flame. If the liquid pool or spill is formed at ambient conditions sufficient to vaporize enough fuel to form a flammable air/fuel mixture, then a flame can propagate through the mixture above the spill as a premixed flame. 

The next step involves heating the mold while it is rotating. Molds can be heated by a heated oven, a direct flame, a heat-transfer liquid (either in a jacket around the mold or sprayed over the mold), or electric-resistance heaters placed around the mold. With uniform heat transfer through the mold, the plastic melts to build up a layer of molten plastic on the molds inside surface. 

The mechanisms that affect heat transfer in single-phase and two-phase aqueous surfactant solutions is a conjugate problem involving the heater and liquid properties (viscosity, thermal conductivity, heat capacity, surface tension). Besides the effects of heater geometry, its surface characteristics, and wall heat flux level, the bulk concentration of surfactant and its chemistry (ionic nature and molecular weight), surface wetting, surfactant adsorption and desorption, and foaming should be considered. 

As shown in Fig. 5.42 an increase in superficial liquid velocity involves an increase in heat transfer (Nul). This effect falls off with increasing superficial gas velocity in the range Reos = 4.7-270. 

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