lab report: force convection

OBJECTIVE

To demonstrate the effect and the use of finned surface and pinned surface to improve the heat transfer in forced convection.

INTRODUCTION

Convection is the movement of molecules within fluids (i.e. liquids, gases). It cannot take place in solids, since either bulk current flows or significant diffusion can take place in solids. Convection is one of the major modes of heat transfer and mass transfer.

Forced convection is a mechanism, or type of heat transport in which fluid motion is generated by an external source (like a pump, fan, suction device, etc.). It should be considered as one of the main methods of useful heat transfer as significant amounts of heat energy can be transported very efficiently and this mechanism is found very commonly in everyday life, including central heating, air conditioning, steam turbines and in many other machines. Forced convection is often encountered by engineers designing or analyzing heat exchangers, pipe flow, and flow over a plate at a different temperature than the stream (the case of a shuttle wing during re-entry, for example). However, in any forced convection situation, some amount of natural convection is always present whenever there are g-forces present (i.e., unless the system is in free fall). When the natural convection is not negligible, such flows are typically referred to as mixed convection.

The removal of excessive heat from system components is essential to avoid damaging effects of burning or overheating. Therefore, the enhancement of heat transfer is an important subject of thermal engineering. Extended surfaces (fins) are frequently use in heat exchanging devices for the purpose of improve the heat transfer between a primary surface and the surrounding fluid. Various types of heat exchanger fins ranging from relatively simple shapes, such as rectangular, square, cylindrical, annular, tapered or pin fins, to a combination of different geometries, have been used. The study of improving heat transfer performance is referred to as heat transfer augmentation, enhancement or intensification.

The heat transfer augmentation is very important subject in industrial heat exchangers and other thermal application. Extended surfaces, which are popularly known as fins, are extensively used in air-cooled automobile engines and in air-cooled aircraft engines. Fins are also used for the cooling of computer processors, and other electronic devices. In various applications heat from the fins is dissipated by natural as well as forced convection and radiation. Fins are used as arrays in all the applications.

Thus, our experiment is to demonstrate the use of a fin (extended surface) to improve the heat transfer in forced convection. More about this was explained details in this report.

THEORY

Heat transfer from an object can be improve by increasing the surface area in contact with the air by adding fins or pins normal to the surface. This can be seen in Newton’s Law of Cooling that states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings, which defines the convection heat transfer rate.

The constant of proportionality h is termed the convection heat-transfer coefficient. The heat transfer coefficient h is a function of the fluid flow, so, it is influenced by the surface geometry, the fluid motion in the boundary layer and the fluid properties as well. The effect of the surfaces can be demonstrated by comparing finned and unfinned surfaces with a flat plate under the same conditions of power and flow.

A heated surface dissipates heat to the surrounding fluid primarily through a process called convection. Heat is also dissipated by conduction and radiation, however these effects are not considered in this experiment. Air in contact with the hot surface is heated by the surface and rises due to reduction in density. The heated air is replaced by cooler air, which is in turn heated by the surface, and rises. This process is called free convection.

Convection heat transfer from an object can be improved by increasing the surface area in contact with the air. In practical it may be difficult to increase the size of the body to suit. In these circumstances the surface area in contact with the air may be increased by adding fins or pins normal to the surface. These features are called extended surfaces. A typical example is the use of fins on the cylinder and head on an air-cooled petrol engine. The effect of extended surfaces can be demonstrated by comparing finned and pinned surfaces with a flat under the same conditions of power input and airflow.

Forced convection is a mechanism, or type of heat transport in which fluid motion is generated by an external source (like a pump, fan, and suction device,). It should be considered as one of the main methods of useful heat transfer as significant amounts of heat energy can be transported very efficiently and this mechanism is found very commonly in everyday life, including central heating, air conditioning, steam turbines and in many other machines. Forced convection is often encountered by engineers designing or analyzing heat exchangers, pipe flow, and flow over a plate at a different temperature than the stream. However, in any forced convection situation, some amount of natural convection is always present whenever there are g-forces present (unless the system is in free fall). When the natural convection is not negligible, such flows are typically referred to as mixed convection.

EQUIPMENTS

The surfaces are shown in the figure below. The finned surface consists of 9 fins that are each 0.1 m high and 0.068 m wide. The pinned surface consists of 17 pins that each have a diameter of 0.013 m and are 0.068 m long.

1. Bench top unit with holder
2. Sensors for measuring temperature and flow velocity
3. Air duct
4. "cylinder" heating element
5. Temperature sensor
6. Measuring glands
7. Fan
8. "finned" heating element
9. "flat plate" heating element
10. Display and control unit
11. Handheld sensor to measure airflow velocity

REFERENCES

1. T.D. Eastop, A. McConkey, Applied Thermodynamics For Engineering Technologists 5^th Edition, Pearson Prentice Hall, 1993.
2. Yunus A.Cengel, Heat and Mass Transfer, Third Editions (SI Units) Mc Graw Hill.
3. Yunus A. Cengel, Michael A. Boles, Thermodynamics: An Engineering Approach 5^th Edition, Mc Graw Hill, 2006.
4. www.en.wikipedia.org