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Unit 13 Principles of Heat Transfer

Practically all the operations that are carried out by the chemical engineer involve the production or absorption of energy in the form of heat. The laws governing the transfer of heat and the types of apparatus that have for their main object the control of heat flow are therefore of great importance.

实际上，所有的由化学工程师进行的操作都要涉及热量的产生和吸收。因此，控制传热的定律和以控制热流为主要目的的仪器类型都是很重要的。 1. Nature of Heat Flow

When two objects at different temperatures are brought into thermal contact, heat flows from the object at the higher temperature to that at the lower temperature. The net flow is always in the direction of the temperature decrease. The mechanisms by which the heat may flow are three, conduction, convection, and radiation.

当两种不同温度的物体开始接触后，热流就会从高温物体传给低温物体。净热流总是随着温度降低的方向。传热的机理通常分三种：热传导，热对流，热辐射。

Conduction If a temperature gradient exists in a continuous substance, heat can flow unaccompanied by any observable motion of matter. Heat flow of this kind is called conduction. In metallic solids, thermal conduction results from the motion of unbound electrons, and there is close correspondence between thermal conductivity and electrical conductivity. In solids which are poor conductors of electricity, and in most liquids, thermal conduction results from the transport of momentum of individual molecules along the temperature gradient. In gases conduction occurs by the random motion of molecules, so that heat is \from hotter regions to colder ones. The most common example of conduction is heat flow in opaque solids, as in the brick wall of a furnace or the metal wall of a tube.

如果在物质表面存在温度梯度，那么热量能够不伴随着任何可观察的运动形式而流动。这种热量流动叫做热传导。在金属体中，热传导是由于自由运动的电子产生的，因此在导热和导电之间存在着相似性。在不导电的固体以及大多数的液体中，热传导是由个别分子沿着温度梯度传递而产生的。在气体中，传热主要是由分子的无规则的运动而产生的，所以热流是由热区域扩散到低温区域。象炉子的砖壁和管件的金属壁这样的不透明固体中的最常见的传热形式是热传导。

Convection When a current of macroscopic particle of fluid crosses a specific surface, such as the boundary of a control volume, it carries with it a definite quantity of enthalpy. Such a flow of enthalpy is called a connective flow of heat or simply convection. Since convection is a macroscopic phenomenon, it can occur only when forces act on the particle or stream of fluid and maintain its motion against the forces of friction. Convection is closelyassociated with fluid mechanics. In fact, thermodynamically, convection is not considered as heat flow but as flux of enthalpy. The identification of convection with heat flow is a matter of convenience, because in practice it is difficult to separate convection from true conduction when both are lumped together under the name convection. Examples of convection are the transfer of enthalpy by the eddies of turbulent flow and by the current of warm air flowing across and away from an ordinary radiator.

当流体的宏观粒子流通过某一特定表面时，例如通过一控制容积的边界，总是带有一定数量的焓。这种焓的流动就叫做连续热流或简称对流。由于对流是一种宏观现象，因此，只有当力作用在微团或液流上且该力能够克服摩擦力并维持其运动时，这种传递现象才能发生。热流常和流体力学联系起来。实际上，从热力学角度，对流常被认为是焓通量而不是热流量。另外，很难将对流和纯传导截然分开，因此，将二者概括起来称为对流时，热对流的识别就很方便了。对流的例子是焓的传递，这些焓是通过湍流的漩涡以及热空气流经和远离传统散热器的涡流的漩涡来传递的。

Natural and forced convection The forces used to create convection currents in fluids are of two types. If the currents are the result of buoyancy forces generated by differences in density and the differences in density are in turn caused by temperature gradients in the fluid mass, the action is called natural convection. The flow of air across a heated radiator is an example of natural convection. If the currents are set in motion by the action of a

mechanical divide such as a pump or agitator, the flow is independent of density gradients and is called forced convection. Heat flow to a fluid pumped through a heated pipe is an example of forced convection. The two kinds of force may be active simultaneously in the same fluid, and natural and forced convection then occur together.

自然和强制对流 在流体中被用来产生对流的方式主要有两种。如果湍流是由于密度差异引起的浮力而产生的，并且这个密度的差异是流体的温度梯度造成的，那么这个运动就是自然对流。空气通过被加热的散热器就是一个自然对流的例子。如果流动是由象泵和搅拌器这样的分离器械的运动所产生的，而与密度差无关，那么这种对流称为强制对流。用泵抽吸流体穿过加热管的热的流动就是一个强制对流的例子。若这两种效应同时作用在流体上，那么自然对流和强制对流就会一起出现。

Radiation Radiation is a term given to the transfer of energy through space by electromagnetic waves. If radiation is passing through empty space, it is not transformed into heat or any other form of energy nor is it diverted from its path. If, however, matter appears in its path, the radiation will be transmitted, reflected, or absorbed. It is only the absorbed energy that appears as heat, and this transformation is quantitative. For example, fused quartz transmits practically all the radiation that strikes it. a polished opaque surface will absorb most of the radiation received by it and will transform such absorbed energy quantitatively into heat.

辐射 若能量传输是以电磁波的形式穿过空间则就用到了辐射这个术语。如果辐射是通过真空的，那么，它不会转化成热量或其他任何形式的能量，也不会发生方向转变。然而，如果在它的传播路径上有物体存在，那么辐射就会被传递，反射或吸收。而只有被吸收的能量才会以热的形式存在，并且这种转换是等量的。例如，融化的石英实际上可以传递所有接触它的辐射，抛光的不透明表面能吸收大多数它所接受的辐射，并且将这些吸收的能量等量地转换成热量。

Monatomic and diatomic gases are transparent to thermal radiation, and it is quite common to find that heat is flowing through masses of such gases both by radiation and by conduction-convection. Examples are the loss of heat from a radiator or unlagged steam pipe to the ambient air of the room and heat transfer in furnaces and other high-temperature gas-heating equipment. The two mechanisms are mutually independent and occur in parallel, so that one type of heat flow can be controlled or varied independently of the other. Conduction-convection and radiation can be studied separately and their separate effects added together in cases where both are important. In very general terms radiation becomes important at high temperatures and is independent of the circumstances of the flow of the fluid. Conduction-convection is sensitive to flow conditions and is relatively unaffected by temperature level.

热辐射能穿透单原子和双原子气体，并且可以发现热量流过这种气体常是辐射和传导—对流同时进行。象散热器或未保温的蒸汽管向房屋周围空气的热量损失及熔炉中的热交换和其他高温气体加热设备。两种机理互相独立同时出现，所以一种热流能独立于另外一种而被控制或改变。传导—对流和辐射可以分开研究，当在两种方式都很重要状态下，可以把它们的独立作用效果加起来。一般而言，高温时辐射占重要地位，而与流体流动的情况没有关系。传导—对流对流动的条件很敏感而相对来说不受温度条件的影响。 2. Rate of Heat Transfer

Heat flux Heat-transfer calculations are based on the area of the heating surface and are expressed in Btu per hour per square foot (or watts per square meter) of surface through which the heat flows. The rate of heat transfer per unit area is called the heat flux. In many types of heat-transfer equipment the transfer surfaces are constructed from tubes or pipes. Heat fluxes may then be based either on the inside area or the outside area of the tubes. Although the choice is arbitrary, it must be clearly stated, because the numerical magnitude of the heat fluxes will not be the same for both.

Average temperature of fluid stream When a fluid is being heated or cooled, the temperature will vary throughout the cross section of the stream. If the fluid is being heated, the temperature of the fluid is a maximum at the wall of the heating surface and decreases toward the center of the stream. If the fluid is being cooled, the temperature is a minimum at the wall and increases toward the center. Because of these temperature gradients

throughout the cross section of the stream, it is necessary, for definiteness, to state what is meant by the temperature of the stream. It is agreed that it is the temperature that would be attained if the entire fluid stream flowing across the section in question were withdrawn and mixed adiabatically to a uniform temperature.The temperature so defined is called the average or mixing-up stream temperature.

(Selected from:Warren L.McCabe,Unit Operations of Chemical Engineering,5th Edition McGraw-Hill Inc.,1993)