Heat Transfer from Extended Surfaces

Heat Transfer from Extended Surfaces

Heat transfer from an expanded surface

Introduction

The term expanded surface is usually used to calculate a specific case of heat transfer; heat transfer by conduction within solid motion and heat by convection (and/or radiation) from the confines of the solid.

In contrast to the previously studied concepts, for an expanded surface, the direct  heat transfer from the boundaries is perpendicular to the main heat transfer direction in the solid.

So finally will, we have to consider the transfer of heat from the solid boundaries to be in the exact same direction as heat transfer by conduction in the solid.

In the study of heat transfer, surface wings extend from an object to increase the rate of heat transfer to or from the environment by increasing convection. The degree of conduction, convection, or radiation of an object determines the amount of heat transferred. Now we'll be Increasing the gradient of temperature between the environment and the object, increasing the convection heat transfer coefficient, or increasing the surface area of ​​the object to increase heat transfer. Sometimes it is not possible or economical to change the first two options. So fining something increases the surface area and can sometimes be an economical solution to heat transfer problems.

Consider a strut that joins two walls at different temperatures and flows water. With this (T1=> T2), temperature gradients in the same x-direction maintain heat transfer by conduction. with T1> T2> T ∞, convection flow to the stream by convection. The magnitude of the temperature gradient will increase, accordingly, by increasing x. This is very important in a number of applications, such as wings (Finenhanced heat transfer).

Improved heat transfer Fin

 Two methods of increasing the rate of heat transfer in Figure 3.13a: Increasing fluid velocity to  increase the convection coefficient h. The fluid temperature may decrease T∞. Increasing the surface area  in which the convection occurs by employing wings

General Behavior Analysis

It is important to first know the extent to which expanded surfaces or final arrangements may improve heat transfer from the surface to the surrounding beer. To find this out, the temperature circulation next to the wing should be determined by considering some assumptions:

 1. Unilateral conduction position in the x-direction.

 2. The conduction rate at any point is equal to the convection rate at that point.

3. The temperature is the same throughout the thickness of the wing (there is the only action to x).

 4. Stable state condition.

5. The thermal conductivity is stable.

6. The radiation from the surface is very small.

7. No heat was generated.

8. Convection heat transfer coefficient (h) uniformity over the surface.

Each shield is attached to the bottom surface of temperature T (0) = Tb and extends into the temperature current T∞.  is stable. As is equal to P.x, where P is a margin

. Fine of Cross-Regional Area

 Example-1

 A very long rod has one end 5 mm in diameter at 100 ° C. The surface of the rod is exposed to ambient air at 25 ° C with a convection heat transfer coefficient of 100 W / m2. K.

Calculate the temperature distributions on rods constructed of real copper,  2024 aluminum alloy, and type AISI 316 steel.

 What are the corresponding losses from the heat to the rods? (Tell in comment)

Fine of Cross-Regional Area

 Example-1

 1. Stable state condition.

 2. One-way movement across the rod.

3. Sustainable buildings.

4. Neutral radiation exchange with approx.

5. Uniform heat transfer coefficient.

6. Endless long rod.

General issue

To create a coherent equation for fine heat transfer, many assumptions must be made:

1. Stable state

2. Buildings of durable materials (independent of temperature)

3. No internal heat generation

4. One-way movement

5. Uniform cross-sectional area

6. Uniform convection over the surface area

Performance

Fin’s performance can be explained in three different ways. The first is efficiency. The rate of fin heat transfer rate is the ratio of the rate of heat transfer of an object if it did not have a wing.

where the cross-sectional area is at the bottom. Fin performance can also be characterized by fin efficiency. This is the ratio of the fin heat transfer rate to the fin heat transfer rate if the entire wing were at the base temperature,

this equation is equal to the surface area of ​​the shield. The efficiency of the shield will always be less than one, because assuming that the temperature across the wing at the base temperature would increase the rate of heat transfer.

Infinite fines (caves)

Open caverns are defined as the segments formed between adjacent wings and stand for the necessary promoters of boiling or nucleate density. These caves are usually used to extract heat from a number of heat-generating bodies. From 2004 to the present, many researchers have been inspired to design the best caves.

Customs

Heaters fine are typically used in mainly heat exchangers as such as radiators in cars and mostly in computer's CPU heat&sinks, and heat exchangers in power plants. They will also be used in newer technology such as hydrogen fuel cells. Nature has also taken advantage of fine onions. The ears of jackrabbits and fennec foxes act as wings to release heat from the blood flowing through them.

Solutions

The base of the wing is usually set to a constant reference temperature, Four tip conditions are possible, however: the tip can be exposed to convective heat transfer, insulated, maintained at a constant temperature, or as long as depart from the base to reach the ambient temperature.

References

1. Lienhard, John H. IV; Lienhard, John H. V. (2019). Heat Transfer Textbook (5th ed.). Mineola, NY: Dover Pub.

2. Lorenzini, G .; Biserni, C .; Rocha, L.A.O. (2011). "Geometric optimization of isothermal caves according to Bejan's theory". International Journal of Heat and Mass Transition. 54 (17–18): 3868–3873. doi: 10.1016 / j.ijheatmasstransfer.2011.04.042.

3. "Finiator machine or machine". Festool International. Received 2006-09-18.

4. “Design of card heat exchangers”. Chart. Originally Archived on 2006-10-11. Received 2006-09-16.

5. "VII.H.4 Development of a Thermal and Water Management System for PEM Fuel Cells". Pont Guillermo. Received 2006-09-17.

6. Cnoc, R .; Veghte, J. (1945. "ears Jackrabbit: surface temperature and Science. responses vascular ".  194 (4263): 436–438. Bibcode: 1976Sci ... 194..436H. DOI: 10.1126 / science.982027. PMID 982027.

•  Frank Incropera; DeWitt,  P David .; Bergman, Theode L .; Lavine, Adrienne S. (2007). Concepts of heat and mass transfer (6 ed.). New York: John Wiley & Sons. pp. 2–168. ISBN 978-0-471-45728-2.