Principle

By suspending a small quantity of water droplets in a gas stream, convective cooling of heated bodies is remarkably improved in comparison with singlephase gas cooling.

Object

This experiment demonstrates how convective heat transfer can be enhanced due to the evaporation of a thin water film, maintained on a heated surface by continuous impingement of water droplets, and the sensible-heat cooling by droplet impingement. The enhancement of heat transfer is governed by the temperatures of the air–water mist and a heated body, air-stream velocity, water-to-air mass flow ratio, size and spatial distribution of water droplets, wet-area fraction of the body surface, and so on.

Apparatus

The outlines of the air–water mist flow tunnels to be used for the heattransfer experiments are illustrated in Figs. 15.1 and 15.2. Figure 15.1 shows a test tunnel of the open type for horizontal air–water mist flow;5 Fig. 15.2 shows a test tunnel of the circulation type for vertical air–water mist flow.2 In the test tunnel shown in Fig. 15.1, pressurized water is sprayed through ten hollow-cone spray nozzles which are arranged in a circle around the centerline of the spray chamber. The additional use of a flat spray nozzle is recommended to humidify and saturate the carrier air. It is better to locate these nozzles sufficiently upstream so that drops larger than the desired ones may settle out of the mist in the low-velocity spray chamber.

Principle

An experiment that involves the measurement of thermostatic properties is described. Temperatures and pressures of a real gas are measured in order that v(T, P) can be constructed. In addition the saturation pressure as a function of temperature Ps(T) is measured so that heats of vaporization can be evaluated.

Object

The primary objective of this experiment is the determination of v(T, P) and Ps(T) over a specified range of temperatures and pressures. In addition, other goals include the: (1) comparison of the measured v(T, P) data with published data and with v(T, P) functions obtained from the principle of corresponding states, and (2) comparison of the measured Ps(T) data with published data and the approximation ln (Ps(T)) = m/T + b.

Background

Most processes, devices, and systems built by engineers utilize gases and liquids. Such analysis and design invariably requires the evaluation of various thermostatic properties. For gases all thermostatic properties can be determined from knowledge of the “mechanical” equation of state v(T, P) and either the “thermal or caloric” equation of state (U = f(T, P)) or cp(T), which is the perfect-gas heat capacity. In addition to these functions, knowledge of Ps(T) provides a means of evaluating the thermostatic properties for two-phase materials. Finally, excursions into the liquid region also require that the heat capacity of the liquid be known.

Principle

The velocity profile in the laminar flow of a fluid through a cylindrical tube is employed to disperse an injected solute. The radial diffusion of the solute in the tube, arising from the radial concentration gradients so created, opposes this dispersion. The combined effect is to produce a solute distribution in the longitudinal direction within the tube that is Gaussian and whose second central moment is related to the mutual diffusion coefficient of the fluid system.

Object

The object of the experiment is the measurement of the mutual diffusion coefficient of a binary liquid mixture. The experiment illustrates how an understanding of fluid mechanics and transport processes can be employed to develop a powerful and simple measurement technique. The aim of the present simple experiment to be described is to determine the diffusion coefficient of the system n-hexane/n-heptane for almost pure n-heptane at 25 °C.

Background

The process of diffusion is the name applied to the relative motion of molecular species in a fluid mixture under the influence of a gradient of concentration, or more completely, of chemical potential. The simplest possible realization of the process is illustrated in Fig. 21.1, where at time t = 0 a concentration difference is established in a binary fluid mixture across an interface at z = 0.

This book represents a collection of the favorite experiments in heat transfer and thermodynamics of some of the world's eminent scholars in the field. It was my intent to include experiments that they had used in their lectures which had a proven track record in regard to students' reaction and understanding. Experiments that were too complicated, even bordering on research, or too esoteric were rejected.

Some of the experiments are well known, some of them are relatively new; and for all of them I have tried to devise the presentation that appears to be best from a consistent point of view. The book has been prepared as a connected account; each experiment is intended to be used as a whole rather than in parts. The readers were envisioned as both engineering and physics students; however, the appeal will probably lie with engineering students from all disciplines.

A brief word is necessary about the selection of topics to be found in this book as well as the order in which they were placed. The experiments selected were varied, representative, new, old, fascinating, and challenging. I selected those I thought my students would enjoy as well as students in Japan, Germany, Brazil, and other countries with different backgrounds and methodologies.

Principle

One method of measuring thermal diffusivities of different solids is to immerse a sphere of the material in a hot (or cold) water bath, and to measure the temperature response at different points within the solid.

Objective

To provide a simple undergraduate experiment illustrating Fourier's Law for unsteady heat conduction.

Apparatus

A 5–8-cm-diameter sphere is used of the material whose thermal conductivity it is desired to measure. Plexiglas is good because one can see the placement of the thermocouples. Other possible choices include wood, rubber, sponge rubber, or even such familiar objects as an apple or an orange. For soft materials sheathed, hypodermic-type thermocouples are preferable. For hard materials small radial holes are drilled to the center and to the midradius. Thermocouples are inserted into the bottom of these holes, and good thermal contact is ensured by using conductive heat transfer paste. The holes are sealed against water entry by silicone or other sealant. A constant-temperature water bath with a stirrer is required, together with a frame to hold the sphere. In the simplest version the thermocouples are read manually from a potentiometer with a thermocouple switch, or from two potentiometers. A preferable arrangement uses amplifiers for the two inserted thermocouples, with digital or analog readout. A multichannel temperature scanner may be used, or the instructor may wish to write a simple program for sampling the data and storing in a PC.

Principle

An image of an electrically heated wire boiling under water is projected on a screen. Boiling in real time is easily observed.

Object

This experiment provides a greatly enlarged, easily controlled demonstration of boiling. The image is so clear the audience leaves with an unforgettable picture of the processes that constitute boiling.

Apparatus

Power is brought into the projector through the posts that are fastened to the lid of the slide. (The lid is shown in profile in Fig. 19.1.) The posts are fastened to leads that are made of springy copper strips with clips on the ends which hold the wire from which the boiling occurs. When the whole device is assembled it looks like Fig. 19.1.

The wire is the only part of the assembly that is critical. We have found that Chromel A wire 0.010 inches in diameter is appropriate. This wire passes through critical heat flux (CHF, burnout, boiling transition, etc.) when an AC current of 6.4 amps passes through it. Very little more current than this also causes the wire to physically burn out.

The power for this wire is provided by a Variac which has a maximum output of 10 amps. An ammeter in the line passing the Variac output is needed to insure that the maximum allowable current is not exceeded.

Principle

At the ice–water interface in a steady-state condition, the heat flux transferred from water to the ice–water interface is equal to that conducted from the ice–water interface to ice. By measuring the coordinates of the ice–water interface, the heat flux from the interface to ice is calculated by the boundaryelement method. The local heat-transfer coefficient on the ice–water interface is, then, estimated by Newton's law of cooling.

Object

Ice formation around tubes in a water flow relates to many practical problems such as lowering thermal efficiency or increasing pressure drop in a water-cooled heat exchanger in a refrigeration system. It also relates to many other applications such as the ice-bank method in a low-temperature heatstorage system. In those cases, the local heat-transfer coefficient on the ice surface is an important factor in predicting the ice amount around the tubes and also the thermal efficiency of the heat exchanger. The measurement of the local heat-transfer coefficient, therefore, presents essential information for practical designs.

Apparatus

The experimental apparatus consists of a calming section, a test section, a flow meter, a refrigeration unit, and two circulation systems of water and a coolant as shown in Fig. 13.1. In Fig. 13.2, a schematic illustration of the test section is shown. The test section has a 0.15-m × 0.04-m cross-sectional area and has a 1.0-m length. The walls are made of transparent acrylic resin plates in order to observe the growth of the ice layer, and are installed in the vertical position to minimize the effect of the natural convection of water.

Thermodynamics is one of the major branches of physics. It is concerned with the behavior of energy as affected by changes of temperature. In particular, thermodynamics explains the observed properties of matter at any temperature. In this connection, we might consider heat capacities, magnetic and electrical effects, phase transitions, and higher-order transitions (such as the Ehrenfest third-order transition) as principle topics.

Classical thermodynamics on the other hand treats the many observable properties of solids and fluids in such a manner that they can all be viewed as a consequence of a few. The four laws of thermodynamics are the result of observation: thus the importance of experimentation in this science. The development of the four laws is elegant. The laws contain an aesthetic spirit that once grasped and understood by the student will stand as the undercurrent for all the other physical sciences.

To tickle the student's imagination consider the application of thermodynamics to one aspect of the study of black holes. It is known a black hole has entropy. For example, the area of the event horizon of a black hole is entropy. Adding mass to a black hole increases the event horizon since it has added entropy. If the black hole has entropy it has temperature, which means black holes can radiate energy. The question arises how can black holes (possessing temperature) emit particles of radiant energy if nothing can escape past the event horizon?

Principle

The temperature response of the unsteady inlet temperature varying with time is an essential parameter in the thermal design of the heat exchanger and other heat transport equipment. The temperature amplitude variation along the general passage decays exponentially and can usually be determined by experiment. This variation will affect the entire heat transport process within the heat exchanger and other heat-transport equipment.

Objective

The objective of this experiment is to determine the decay of the temperature oscillation along the channel for a timewise oscillation of inlet temperature. In practical applications, the heat transfer within the channel may be exposed to some planned or unplanned transients or start-ups and shutdowns during the operation. Thus, such a knowledge is critically necessary for those devices which never attain steady-state operation because of their nature of periodical operation in time.

Apparatus

The experimental apparatus consists of a rectangular duct with different sections of filter, calming, inlet, test, and convergence. The geometry of the test section is a rectangular duct with a cross section of 254 × 25.4 mm2 (10 × 1 in.2). The instrumentation includes a wave generator, a power supply, a heater, an inclined manometer, voltmeters, thermocouples, an orifice plate, and a fan, as shown in Fig. 14.1.

Air flows from the calming section to the inlet section (2770 mm in length) wherein the velocity becomes fully developed.

The following list of experiments and demonstrations is presented to supplement those given in Part I of the book. The experiments have varying degrees of difficulty and should offer additional variety to illustrate the fundamentals of heat transfer.

Background

Engineering students are unique. They are usually uninterested in a problem unless they can visualize it. There are two ways visualization can be accomplished. One can create a mathematical model where mathematical symbols simulate properties, devices, and behaviors, or one can create the engineering problem in the laboratory. The former is usually faster and easier for the teacher, and the latter appears to be disappearing from educational institutions due to the ease and familiarity with the computer.

Motivation

The motivation behind this book is based on three quotations:

Surely excellence in instruction is at the very root of education, and of necessity demands the maintenance of good academic standards. In that light, it follows that performing experiments is the grist of engineering. Nothing can be more significant than the marriage of excellent instruction incorporating well-defined academic standards with student involvement in the laboratory, that is, having the student put that instruction to practical use.

Principle

Transfer of electricity out of a storage battery is much more efficient (closer to reversibility) when it is very fast rather than very slow.

Object

It is often argued that reversible processes take an infinite time to complete and, therefore, are of questionable usefulness. Although there is truth in this argument for certain processes, such as transfer of energy across a finite temperature difference, the argument is neither universally valid nor representative of some practical phenomena. A simple and very important counterexample is provided by a storage battery. If discharged quickly, the battery does work almost equal to the stored energy. If discharged more slowly than the rate of its internal discharge (let alone infinitely more slowly), the battery does practically no work, that is, all its availability is dissipated. The availability is dissipated because the internal discharge generates entropy spontaneously or, said differently, the internal discharge is irreversible.

Apparatus

As shown schematically in Fig. 29.1, the apparatus consists of a cell with two electrodes (1), a temperature bath (2), a glass thermometer and an electric heating plate (3), a galvanostat (4), a resistance box (5), an ammeter (6), and a voltmeter (7). Another schematic of the apparatus is shown in Fig. 29.2.