The purpose of this lab is to understand natural and forced convection on a cylinder by measuring surface and ambient temperatures and relating the data to convection heat transfer equations.
AIMS & OBJECTIVES
Objectives To investigate Free Convection and Radiation 1. Determine the emissivity (of an element experimentally. Determine the Heat transfer coefficients by free convection Theory Natural Convection: Heat transfer through the circulation of fluid due solely to gravity.
Forced Convection: Heat transfer through the circulation of fluid due to forced fluid movement (fan, pump, etc. ) Radiation: Heat transferred by surface photon emission, typically only significant at T>>Room Temp. EXPERIMENT Apparatus Used Figures 1 below shows the vacuum pump vessel and measuring equipment used The apparatus consisted of a heated element which was suspended inside a [pressure vessel. The air pressure in the vessel was varied by the use of either a bleed valve or a 240v vacuum pump. The heat input to the element was varied by up to 10W, the max working temp was not to exceed 200°C and maintained at that temperature or less throughout the experiment. The heat, power Input, the element, vessel temperatures and the air pressure inside the vessel was determined by the instruments provided for the experiment
Using the wall-mounted barometer the atmospheric pressure was 1018 MB The gauge gives a reading of gauge pressure (diff between the pressure inside the vessel and pressure outside the vessel). Absolute pressure (P) = pressure gauge reading + atmospheric pressure (MB)
Pressure reduced to 2mB and input voltage set to 8. 21 volts.
Observations and readings were taken after 15 mins to allow the system to stabilize and readings tabulated.
Item 3 repeated with Vacuum pressure reduced by 12, 60, 200, 500, and then finally with the bleed valve fully open tabulated as before.
The bleed valve was then fully opened to allow the pressure inside the vessel to meet atmospheric pressure and readings tabulated.
RESULTS, CALCULATIONS, OBSERVATIONS & CONCLUSIONS
Observations During Tests The initial observations were of the temperature, vacuum pressure, and vessel pressures in relation to the inside diameter of the vessel and element assembly. The Temp Diff verses Abs pressure graph below (Graph 1) shows the temp difference at zero free convection given by the equation for a straight line Y=MX+C Surface area of the vessel was given as 3070mm? , Element Length was given as 152mm and 6. 35mm respectively. The following Tables detail what is actually occurring to temperature and heat transfer inside the vessel.
Absolute Pressure calculation Abs Press = Gauge pressure – Atmos Pressure =1015Mb – 1018Mb = 3^1/4 Graph of Pressure Against Temp Difference [pic] Graph 1 Conclusions Temp difference for free convection crosses Y-axis is at 160(K) for zero gas pressure, the power by the heater element has transferred completely to the vessel by radiation at his point. Natural convection is more prevalent at lower temperatures whereas radiation is more prevalent at higher temperatures
Possible Sources of error:
conduction from the heated cylinder to its housing tube
possible changes in ambient temperature
variations in surface temperature
Heat Transfer by Convection and uses Heat typically does not flow through liquids and gases by means of conduction. Liquids and gases are fluids; their particles are not fixed in place; they move about the bulk of the sample of matter. The model used for explaining heat transfer through the bulk of liquids and gases involves convection. Convection is the process of heat transfer from one location to the next by the movement of fluids. The moving fluid carries energy with it. The fluid flows from a high-temperature location to a low-temperature location. (Images courtesy Peter Lewis and Chris West of Standford’s SLAC. ) To understand convection in fluids, Consider the heat transfer through the water that is being heated in a pot on a stove. The source of the heat is the stove burner. The metal pot that holds the water is heated by the stove burner. As the metal becomes hot, it begins to conduct heat to the water. The water at the boundary with the metal pan becomes hot. Fluids expand when heated and become less dense.
So as the water at the bottom of the pot becomes hot, its density decreases. The differences in water density between the bottom of the pot, and the top of the pot results in the gradual formation of circulation currents. Hot water begins to rise to the top of the pot displacing the colder water that was originally there. And the colder water that was present at the top of the pot moves towards the bottom of the pot where it is heated and begins to rise. These circulation currents slowly develop over time, providing the pathway for heated water to transfer energy from the bottom of the pot to the surface. Convection also explains how an electric heater placed on the floor of a cold room warms up the air in the room. Air present near the coils of the heater warm-up. As the air warms up, it expands, becomes less dense, and begins to rise. As the hot air rises, it pushes some of the cold air near the top of the room out of the way. The cold air moves towards the bottom of the room to replace the hot air that has risen. As the colder air approaches the heater at the bottom of the room, it becomes warmed by the heater and begins to rise. Once more, convection currents are slowly formed.
Air travels along these pathways, carrying energy with it from the heater throughout the room. Convection is the main method of heat transfer in fluids such as water and air. It is often said that heat rises in these situations. The more appropriate explanation is to say that heated fluid rises. For instance, as the heated air rises from the heater on a floor, it carries more energetic particles with it. As the more energetic particles of the heated air mix with the cooler air near the ceiling, the average kinetic energy of the air near the top of the room increases. This increase in the average kinetic energy corresponds to an increase in temperature. The net result of the rising hot fluid is the transfer of heat from one location to another location. The convection method of heat transfer always involves the transfer of heat by the movement of matter. The two examples of convection discussed here – heating water in a pot and heating air in a room – are examples of natural convection. The driving force of the circulation of fluid is natural – differences in density between two locations as the result of fluid being heated at some source. Some sources introduce the concept of buoyant forces to explain why the heated fluids rise. We will not pursue such explanations here. Natural convection is common in nature. The earth’s oceans and atmosphere are heated by natural convection. In contrast to natural convection, forced convection involves fluid being forced from one location to another by fans, pumps, and other devices. Many home heating systems involve force air heating. Air is heated at a furnace and blown by fans through ductwork and released into rooms at vent locations. This is an example of forced convection.
The movement of the fluid from the hot location (near the furnace) to the cool location (the rooms throughout the house) is driven or forced by a fan. Some ovens are forced convection ovens; they have fans that blow heated air from a heat source into the oven. Some fireplaces enhance the heating ability of the fire by blowing heated air from the fireplace unit into the adjacent room. This is another example of forced convection. Heat Transfer by Radiation A final method of heat transfer involves radiation. Radiation is the transfer of heat by means of electromagnetic waves. To radiate means to send out or spread from a central location. Whether it is light, sound, waves, rays, flower petals, wheel spokes, or pain if something radiates then it protrudes or spreads outward from an origin. The transfer of heat by radiation involves the carrying of energy from an origin to the space surrounding it. The energy is carried by electromagnetic waves and does not involve the movement or the interaction of matter. Thermal radiation can occur through matter or through a region of space that is void of matter (i. e. , a vacuum).
In fact, the heat received on Earth from the sun is the result of electromagnetic waves traveling through the void of space between the Earth and the sun. All objects radiate energy in the form of electromagnetic waves. The rate at which this energy is released is proportional to the Kelvin temperature (T) raised to the fourth power. Radiation rate = k•T4 (Images courtesy Peter Lewis and Chris West of Standford’s SLAC. ) The hotter the object, the more it radiates. The sun obviously radiates off more energy than a hot mug of coffee. The temperature also affects the wavelength and frequency of the radiated waves. Objects at typical room temperatures radiate energy as infrared waves. Being invisible to the human eye, we do not see this form of radiation. An infrared camera is capable of detecting such radiation. Perhaps you have seen thermal photographs or videos of the radiation surrounding a person or animal or a hot mug of coffee or the Earth. The energy radiated from an object is usually a collection or range of wavelengths. This is usually referred to as an emission spectrum. As the temperature of an object increases, the wavelengths within the spectra of the emitted radiation also decrease. Hotter objects tend to emit shorter wavelength, higher frequency radiation. The coils of an electric toaster are considerably hotter than room temperature and emit electromagnetic radiation in the visible spectrum. Fortunately, this provides a convenient warning to its users that the coils are hot. The tungsten filament of an incandescent light bulb emits electromagnetic radiation in the visible (and beyond) range. This radiation not only allows us to see, but it also warms the glass bulb that contains the filament. Put your hand near the bulb (without touching it) and you will feel the radiation from the bulb as well.
Thermal radiation is a form of heat transfer because the electromagnetic radiation emitted from the source carries energy away from the source to surrounding (or distant) objects. This energy is absorbed by those objects, causing the average kinetic energy of their particles to increase and causing the temperatures to rise. In this sense, energy is transferred from one location to another by means of electromagnetic radiation. The image at the right was taken by a thermal imaging camera. The camera detects the radiation emitted by objects and represents it by means of a color photograph. The hotter colors represent areas of objects that are emitting thermal radiation at a more intense rate. Conclusion The experiment described above provides a convenient method whereby You may investigate the different processes that contribute to cooling in a standard laboratory experiment. In particular, the measurements obtained to enable you to clarify the relative contributions from convection and radiation. Examples of Free – Natural Convection Heat transfer by natural convection occurs when a fluid is in contact with a surface hotter or colder than itself. As the fluid is heated or cooled it changes its density.
This difference in density causes movement in the fluid that has been heated or cooled and causes the heat transfer to continue. There are many examples of natural convection in the food industry. Convection is significant when hot surfaces, such as retorts which may be vertical or horizontal cylinders, are exposed with or without insulation to colder ambient air. It occurs when food is placed inside a chiller or freezer store in which circulation is not assisted by fans. Convection is important when the material is placed in ovens without fans and afterward when the cooked material is removed to cool in air. Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion (observable movement) of fluids. Heat is the entity of interest being advected (carried) and diffused (dispersed). This can be contrasted with radiative heat transfer, the transfer of energy through electromagnetic waves. Heat is transferred by convection in numerous examples of naturally occurring fluid flow, such as wind, oceanic currents, and movements within the Earth’s mantle. Convection is also used in engineering practices to provide desired temperature changes, as heating of homes, industrial processes, cooling of equipment, etc.
The rate of convective heat transfer may be improved by the use of a heat sink, often in conjunction with a fan. For instance, a typical computer CPU will have a purpose-made fan to ensure its operating temperature is kept within tolerable limits. Typical Examples of Heat Transfer CONDUCTION: Heat conduction is an essential and commonplace part of our daily lives, in industry, and in nature. Whenever heat needs to be transferred through an opaque substance, the transfer must be by conduction. In a hot-water heating system, for example, heat from burning fuel is transferred by conduction through the iron or steel of the boiler to heat the water. The heat from a burner on a stove is conducted through the bottom of utensils to cook food. In nature, the surface of the earth is heated by the sun, and some of this heat is conducted to deeper layers of the soil during the day and back to the surface at night-the varying ability of different kinds of soil and water to absorb and conduct heat received from the sun has a profound effect on local and worldwide weather and climate. Examples Touching a stove and being burned -Ice cooling down your hand -Boiling water by thrusting a red-hot piece of iron into it.
CONVECTION: Free, or natural, convection occurs when bulk fluid motion (steams 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. Examples -Hot air rising, cooling, and falling (convection currents An old-fashioned radiator (creates a convection cell in a room by emitting warm air at the top and drawing in cool air at the bottom).
RADIATION: – Heat from the sun warming your face- Heat from a lightbulb – Heat from a fire – Heat from anything else which is warmer than its surroundings. – Gas chambers in Jet engines – Circulation Boiler Furnaces Industrial example Radiation Heat transfer generally occurs in Higher temperature applications within processes with furnace temperatures above about 2200°F (1200°C). They usually have furnaces that use combustors such as in the metals, minerals, and waste incineration industries. In general, the dominant heat transfer mechanism in those industries is thermal radiation. This is in contrast to lower temperature applications where both radiation and forced convection are often important.
Understanding Physics sections 11. 5 – 11. 7, John Wiley & Sons 1998.
C. T. O’Sullivan, Correction for cooling techniques in heat experiments. Physics Education, 25, 176 – 179 (1990).
The data acquisition system (data logger) used was the Prolab system developed under the Leonardo da Vinci Programme ComLab2 (project NO SI 143008); website www.e-prolab.com/comlab/.
In some situations differences between Ts and Ta may be important; see, for example, C. T. O’Sullivan, Newton’s law of cooling – a critical assessment, Amer. J. Phys. , 58 (10), 956 – 960 (1990).
http://www.roymech.co.uk/Useful_Tables/Manufacturing/Extruding.tml Russ College of Engineering and Technology at Ohio University.