Principle

Existence of two different characteristic regions in nucleate boiling, that is, the region of isolated bubbles at low heat fluxes and the region of interference at high heat fluxes, can be demonstrated by measuring the resultant force acting on a compact heater that is suspended in a pool of liquid with boiling on its upper surface.

Object

As the surface temperature of a heater submerged in a pool of saturated liquid is raised above the saturation temperature, nucleate boiling appears after the incipience of boiling, and then continues up to the point of critical heat flux. In this nucleate-boiling regime, the heat flux from the heated surface to the liquid increases with increasing surface temperature, and the relationship between the heat flux and the surface temperature (the so-called boiling curve) is of a monotonic nature, in most cases exhibiting no noticeable change of character throughout the aforementioned regime. Visual observations by means of a high-speed cine camera, however, reveal that nucleate boiling is divided into two regions. Namely, when the heat flux is low, small isolated bubbles repeat the growth and departure process at a comparatively small number of active nucleation sites on the heated surface, but at high heat fluxes, the heated surface is covered with a thin liquid sublayer (which is generally called the macrolayer) holding numerous tiny vapor jets rooted to nucleation sites, and the vapor fed continuously from these vapor jets accumulates to develop massive vapor slugs successively on the foregoing liquid sublayer.

Principle

A linear heat source placed in a material of semi-infinite nature causes temperature changes due to heat conduction. The relationship between temperature at a given distance from the heat source and time in the coordinate system (T - To, log t) is linear after some time.

Object

The experiment demonstrates heat conduction in semi-infinite space. It allows one also to obtain experimentally both the thermal conductivity and the thermal diffusivity of the material. An object used for the experiment can be any material in the form of powder, granules, or paste in bulk.

Apparatus

The apparatus consists of a glass beaker (diameter 0.1 m, height 0.2 m) which is filled with a selected material. At the center of the beaker a linear heat source 0.1 m long is placed and there is a temperature sensor parallel to the heater at a distance of 10 mm from the heater (Fig. 4.1). Both of them are in the form of a capillary metal tube 0.2–0.5 mm in diameter. The heat source is connected to a stabilized power supply (approximately 1.5 W) and the temperature sensor to a display and/or to a recorder.

Procedure

The experiment is carried out at ambient temperature. Switch on the power and register temperature in 2 minute intervals for 20 minutes. After completing the experiment remove the heat source and the temperature sensor to cool them to ambient temperature.

We talk of heat as energy in the process of being transferred. Note, heat is not stored within matter, but rather heat is either “done on” or “done by” matter. Heat is a way of transferring energy across the boundaries of a system. It should be noted that heat is not a conserved substance, as was thought in the past (a remnant of the caloric theory of heat). Also, it is not a fluid as one might envision when the phrase heat flow is used, nor is heat a property of matter. Thus terms such as “heat of a substance” are meaningless. Since heat is neither a property of a system nor contained in a system, we speak of heat as a mode of energy transfer accompanied by a net amount of entropy transfer uniquely specified by the energy transfer as well as the temperature at which it occurs.

We may transfer heat by three different modes: conduction, convection, and radiation. Since each mode is subject to different laws, experiments such as those contained herein are necessary in order to understand the physical aspects involved in a heat-transfer problem.

Object

In the design of continuous-flow combustion systems, an important performance requirement is that combustion must be sustained over a wide range of operating conditions. For the combustors employed in aircraft gas turbines, this poses special problems because they are often called upon to operate at very low inlet temperatures and pressures and at fuel/air ratios that lie well outside the normal burning limits of hydrocarbon–air mixtures.

The stability performance of a continuous-flow combustor is usually expressed in the form of a stability plot that separates the regions of stable and unstable combustion. The traditional plot has equivalence ratio or fuel/air ratio as the ordinate, and some loading parameter, such as air velocity or air mass flow rate through the combustor, as the abscissa. A plot of this type is often called a stability loop, owing to its shape, as illustrated in Fig. 32.1.

Background

Stability loops provide two basic kinds of information. First, for any given fuel/air ratio, they indicate the blowout velocity UBO, which is the gas velocity at which flame extinction occurs. Attention is usually focused on the maximum blowout velocity, which tends to coincide with mixture strengths that lie close to the stoichiometric value. Second, for any given combustor loading, they show the range of fuel/air ratios over which stable combustion can be achieved.

Principle

The lumped-mass assumption (negligible internal thermal resistance) is used to infer forced-, natural-, or mixed-convection heat-transfer coefficients on spheres, cylinders, and other shapes.

Objective

This experiment allows the estimation of heat-transfer coefficients that result from external flows. Use of the lumped-mass approximation is a key element to the work, and this experiment can be used to explore the limits of this important experimental convective technique. Also of value is the estimation of the radiation contribution compared to the convective contribution in the total heat loss from a heated object. The basic approach can be used for a variety of geometries in forced-, free-, or mixed-convection arrangements. The description here focuses on forced-convection applications, but the basic apparatus and concept can be used for the other situations as well.

Apparatus

• Constant-speed centrifugal fan with uniform flow outlet and damper on inlet (for forced-flow experiments)

• Hand-held anemometer, propeller type (for forced-flow experiments)

• Thermocouple reference junction and signal readout device (many data loggers combine these functions and can be used). A highly desirable alternative is a computer-based data acquisition system.

• Clock (if time is not recorded with data logger or computer)

• Support stand

• Bunsen burner and lighter

• Two thermocouples, one in ambient air and one with adaptor for test element

• Barometer

• Copper test elements (e.g., sphere, cylinder)

A diagram of the test apparatus suitable for forced-flow experiments is shown in Fig. 12.1.

Principle

The annular heat-pipe design can significantly increase the heat capacity per unit length compared to conventional cylindrical heat pipes due to the capillary forces generated in the wick material on the inner pipe.

Object

The primary objective of this experiment is to compare the maximum heat transport (capillary) limit of the annular heat pipe to that of a conventional cylindrical heat pipe with the same outer diameter and wick structure. A secondary objective is to examine the temperature distributions on the inside and the outside of the annular heat pipe. Finally, the problem of condensate leakage between the inner wick and the outer wick will be addressed.

Apparatus

The annular heat pipe, as shown in Fig. 24.1, consists of two concentric pipes of unequal diameters attached by means of end caps, which create an annular vapor space between the two pipes. Wick structures are placed on both the inner surface of the outer pipe and the outer surface of the inner pipe (Fig. 24.2). Axial grooves were chosen in this experiment since no special procedures are needed for installation, but any type of wick can be used. The space inside the inner pipe is open to the surroundings. An increase in performance is expected as a result of the increase in surface area exposed for the transfer of heat into and out of the pipe, and the increase in the cross-sectional area of the wick inside the pipe.

Object

The objectives of this experiment are as follows:

1. (i) to observe the evaporation and boiling phenomena in a drop after it falls onto a heated surface,

2. (ii) to measure the lifetime of the drop, and

3. (iii) to determine the heat-transfer characteristics in the sessile drop-boiling system.

Apparatus

Figure 20.1 is a schematic of the experimental setup. It consists of a dropgenerating system and an electrically heated testing surface with a thermal measurement device.

The liquids used in the drops are carbon tetrachloride, benzene, methyl alcohol, and distilled water.

A liquid fills a bottle equipped with a stopcock which is attached to a support and hangs over the heated surface. A needle valve regulates the rate of dripping flow from the bottle through a no.-1/4 hypodermic needle. The liquid is released from the needle in the form of drops 2 to 3 mm in diameter which fall at regular intervals.

In order to generate drops of identical size, a drop receiver is used to collect the drops until they fall at a steady, desired rate.

At the start of the experiment, the receiver is quickly displaced via the action of a spring to allow only a single drop to fall onto the center of the heated surface.

A center rest-pin is installed on the drop receiver to produce two-dimensional movement (in order to prevent drop-receiver vibration).

The following list of experiments and demonstrations is presented to supplement those given in the body of this book. Though they use different formats than that used herein, there is sufficient information and challenge for students to easily assemble the apparatus and conduct measurements that will assist them in understanding more about thermodynamics.

Principle

The resistance to heat transfer in a liquid–liquid direct-contact heat exchanger is found to exist mostly in the dispersed phase. An exception is the drop formation and release process, where the resistances to heat transfer in the continuous phase and the dispersed phase are often of the same order of magnitude. Temperature measurements combined with high-speed motion photography allow one to determine the dispersed-phase and continuous-phase resistances as well as the heat-transfer efficiency.

Object

The objective of this experiment is to determine the heat-transfer efficiency of the drop formation process, and to find the internal and external heattransfer coefficients immediately following release. The liquid–liquid system described here uses hot water as the continuous phase and cold AMSCO petroleum solvent (oil) as the dispersed phase.

Background

Most liquid–liquid direct-contact heat exchangers consist of a vertical spray column. The dispersed phase, which has a lower density than the continuous phase, is injected at the bottom of the column through a nozzle or a set of nozzles and flows upward in the column in the form of drops. The continuous phase is injected at the top of the column and flows downward.

The problem of finding a nonintrusive method to measure the internal temperatures of a developing drop has not been solved. Some researchers have attempted to measure the average drop temperature in the free-rise section of the spray column by effectively changing the column height and measuring the temperature of the dispersed-phase fluid after coalescence.

Principle

The rate at which heat is transferred from the surface of a heated object will become equivalent to the rate at which heat is generated within the object when steady-state conditions have been attained. If the associated conduction and convection heat-transfer effects can be eliminated, radiation heat transfer will become the only means of transferring the heat.

Object

It is an easy matter to model the heat-exchange process that takes place between a heated sphere and a spherical shell in which it is concentrically located because of the simplicity of the geometry. If the heat flowing along the support by which the heated sphere is suspended is negligible and most of the air has been extracted from the space between the heated sphere and the spherical shell, radiation heat transfer becomes the only mechanism capable of transferring the heat generated within the sphere. The purpose of this experiment is to determine the emissivity of a copper surface by substituting the temperatures of the heated sphere and the glass shell corresponding to steady-state conditions into the radiation heat-transfer equation and performing an analysis that yields the value of emissivity.

Background

Figure 23.1 depicts a copper sphere of radius Rcs suspended within a spherical glass jar of radius Rgj. The sphere is attached to the cover plate by a thin-walled stainless-steel tube which prevents any significant heat transfer by conduction.

Principle

The heat transfer across the air boundary layer that descends along a vertical ice slab causes melting at the surface.

Objective

The effect of heat transfer by boundary-layer natural convection over a vertical wall can be visualized and measured by experimenting with thin slabs of ice suspended vertically in still air. The uneven distribution of heat flux is demonstrated by the uneven thinning of the ice slab. The instantaneous flow rate of meltwater collected under the dripping ice is a measure of the overall heat transfer rate from the ambient to the isothermal surfaces of the slab. An additional objective of this experiment is to show that laboratory apparatuses can be built quite inexpensively, often by using kitchen utensils. This experiment teaches a group of students to critically evaluate each others' data, and to pool all their findings into a comprehensive report that may have engineering significance.

Apparatus

• Baking pan

• Refrigerator

• String

• Cardboard box

• Sheet-metal tray

• Thermometer

• Graduated beaker

• Clock

The heart of the apparatus is a vertical slab of ice, which is suspended by means of a string in still air. The manufacture of the ice slab and its suspension and the maintenance of a nearly motionless and isothermal ambient are the critical aspects of the apparatus construction.

An inexpensive way of producing ice slabs of one or more sizes is to use a flat-bottom baking pan (or cookie sheet) placed horizontally in the freezer of a household refrigerator.

Principle

The process of nucleate boiling from a heated surface involves bubble formation, bubble emission, and liquid replacement in a cyclic manner. The phenomenon is strictly periodic at low heat fluxes, becoming gradually aperiodic as the heat flux increases.

Object

This is an experiment to provide some understanding of the complex phenomena that occur during liquid-to-vapor phase change. The geometry is greatly simplified so that the process can be easily controlled and most of the results visually observed. Periodicity and aperiodicity of the bubbling can be quantitatively analyzed through a study of the bubble departure periods.

Apparatus

Figure 17.1 (not to scale) shows the arrangement used in the experiment. The capillary tube in which boiling is to be studied is constructed in the following manner. A thin electrical heater wire (of constantan, diameter 75 μm, for instance) is uniformly roughened with emery paper to discourage preferential nucleation at any particular spot on its surface. It is then run down the center of a 1–3-mm-diameter glass capillary; a 4–7-cm length of this capillary tube is closed off by heating over a flame. A DC power source supplies variable current to the electrical wire. The power from this source can be determined by measuring its voltage and amperage. The length of the wire should also be determined to provide the heat flux in W/m of the heater length.

Objective

Thermal conductivity is one of the most important thermophysical properties in the evaluation of heat flow rate within a solid. Various methods such as the absolute method, the twin plate method, the hot-wire method, and so forth, so far have been proposed for the thermal conductivity measurement of solids. These conventional methods, however, are not applicable for measurements of thermal conductivity when the materials to be measured are subject to phase change or chemical reaction. It is difficult in these methods to eliminate the additional heat flow released by phase change or reaction from the total heat flow.

A novel method for measuring thermal conductivity of solids in such an unsteady-state accompanied by heat-generation or heat-absorption is described. In this method, the value of the effective thermal conductivity in the process of reaction is evaluated from the integrated time change of the temperature of the reacting solid material (D.T.A. curve), by removing the effect of the reaction heat.

Background

The principle equation for determining the thermal conductivity in the process of reaction can be derived by referring to a D.T.A. (differential thermal analysis) measurement (see Table 5.1).