Physical Mechanism of Convection. Conduction and convection are similar in that both mechanisms require the presence of a material medium.

Transcription

1 Convection 1

2 Physical Mechanism of Convection Conduction and convection are similar in that both mechanisms require the presence of a material medium. But they are different in that convection requires the presence of fluid motion. Heat transfer through a liquid or gas can be by conduction or convection, depending on the presence of any bulk fluid motion. Convection is the process in which heat is carried from place to place by the bulk movement of a fluid. Convection currents are set up when a pan of water is heated. how? 2

3 Convection Hold your hand over and under the flame of a match. What do you notice? Why? Hot air expands, becomes less dense and then rises. Heat is convected upwards.

4 What happens to the particles in a liquid or a gas when you heat them? Liquids and gases can both flow and behave in similar ways, so they are called fluids. What happens to the particles in a fluid when it is heated? heat less dense fluid The heated fluid particles gain energy, so they move about more and spread out. The same number of particles now take up more space, so the fluid has become less dense (i.e. The particles spread out and become less dense)

5 Fluid Movement Cooler (more dense) fluids sink through warmer (less dense) fluids. In effect, warmer liquids and gases rise up. Cooler liquids and gases sink.

6 What is Convection? Warmer regions of a fluid are less dense than cooler regions of the same fluid. The warmer regions will rise because they are less dense. The cooler regions will sink as they are more dense. This is how heat transfer takes place in fluids and is called convection. The steady flow between the warm and cool sections of a fluid, such as air or water is called a convection current.

7 Convection Convection is a process involving mass movement of fluids. The mechanism of convection is the transfer of heat energy by actual physical movement of fluid molecule from one place to another in which there exists a temperature gradient. This mode of heat transfer is comprised of two mechanism energy transfer due to random molecular motion (diffusion) and bulk motion of fluid 7

8 Conceptual Examples: Hot Water Baseboard Heating and Refrigerators Mounted near the top of the refrigerator Mounted on the wall next to the floor Location is designed to maximize the production of convection currents: How? 8

9 buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object.

10 The air above the baseboard unit is heated, like the air above a fire. Buoyant forces from the surrounding cooler air push the warm air upward. Cooler air near the ceiling is displaced downward and then warmed by the baseboard heating unit, leading to the convection current. Had the heating unit been located near the ceiling, the warm air would have remained there, with very little convection to distribute the heat. 10

11 The air in contact with the topmounted coil is cooled, its volume decreases and its density increases. The surrounding warmer and less dense air cannot provide sufficient buoyant force to support the colder air which sinks downward. In the process, warmer air near the bottom is displaced upward and is then cooled by the coil, establishing the convection current. Had the cooling coil been placed at the bottom of the refrigerator, stagnant, cool air would have collected there, with little convection to carry the heat from other parts of the refrigerator to the coil for removal. 11

12 Test Yourselves 1. Usually, it is better to install air conditioner in the higher portion of the wall. Please explain this statement with reasons. 2. How are winds forms? 3. Why is the heating coil of an electric kettle placed near the bottom of the vessel?

13 Application of convection Sea Breeze Land Breeze

14 Sea Breeze During the day the sun heats the land much faster than the sea. The air above the land is heated, expands and rises. Cold air from the sea moves inland to take its place. Hence, sea breeze is obtained. Discussion : How land breeze is produced?

15 Land Breeze At night: Land loses heat faster than the sea. Hot air above the sea which is less dense, expands and rises. Cold air from the land moves towards the sea. Convection current is formed. Land Breeze is obtained.

16 Application of convection Electric kettle The heating element is always placed at the bottom of the kettle. So that hot water at the bottom which is less dense will rise up. Cooler water at the top which is denser will sink to the bottom. Convection current is set up to heat up the water.

17 Convection: The mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion and it involves the combined effects of conduction and fluid motion. The faster the fluid motion, the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction. Convection Heat transfer from a hot surface to air by convection. 17

18 Convection Heat transfer engineering is the predication of the rate of heat transfer between a fluid and solid boundary surface. This take place in various steps. Suppose the solid surface is at higher temperature than the fluid. First, heat will flow by conduction from the surface to adjacent particles of fluid. The energy thus transferred will serve to increase the temperature and the internal energy of these fluid particles. Thus the fluid particles will move to a region of cold fluid (lower temperature in the fluid) where they will mix with and transfer a part of their energy to other fluid particles. The energy is actually stored in the fluid particle and due to mass motion of fluid particles, it is also carried away. Thus the heat transfer between a solid surface and fluid is due to conduction as well as convection. 18

19 Natural (or free) convection: If the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid. e.g. Heat flow from a hot place to atm Heating a room by a stove Forced convection: Force convection is the presence of fluid flow along with heat transfer This fluid flow caused by some external agency such as a fan, pump, blower, stirrer or the wind. e.g. Heat exchange in condenser Air conditioning equipments Types of Convection The cooling of a boiled egg by forced and natural convection Heat transfer processes that involve change of phase of a fluid are also considered to be convection because of the fluid motion induced during the process, such as the rise of the vapor bubbles during boiling or the fall of the liquid droplets during condensation. 19

20 Types of Convection Natural convection, in which a temperature difference causes the density at one place in a fluid to be different from that at another. The forced convection generated by a pump circulates radiator fluid through an automobile engine to remove excess heat. 20

21 Force Convection Flow through a duct or sets of ducts Heat exchanger Gas turbine blades 21

22 Free Convection In natural convection, the fluid velocity far from the solid body will be zero. However, near the solid body there will be some fluid motion if the body is at a temperature different from that of the free fluid. In this situation there will be a density difference between the fluid near the solid surface and that far away from the system. There will be a positive or negative buoyancy force due to this density difference. Hot surface will create positive buoyancy force whereas the cold surface will create the negative buoyancy force. Therefore, buoyancy force will be the driving force which produce and maintain the free convective process. Free convection boundary layer for vertical (a) hot surface and (b) cold surface 22

23 Free Convection: Heat flow from a hot surface to atmosphere Hot Surface at T w immerse or expose to a fluid at temperature T which is away from the wall uninfluenced by its presence There is no bulk flow That means If hot surface were not be at a different temperature then fluid be a stagnant Assume T w > T Fluid is not flowing so because of conduction from the hot surface, layers near the hot surface will have temperature approaching T w Some temperature gradient will be established T w high so ρ 1 < ρ (density in free stream) In bulk fluid there is a zone of low density, these zone of low density leads to buoyancy forces acting on this. and hence, this zone of low density tends to moves up and thus setting up the process natural convection. Its moves up and is replaced by fluid from the bulk and the process established is called 23 natural convection.

24 OR The stagnant layer of air in the immediate vicinity of the plate gets thermal energy by conduction. Thus energy transferred serves to increase the temperature and internal energy of the air particles. Because of temperature rise, these particles become less dense (lighter) than the surrounding air. The lighter air particles move upward to a region of low T where they mix with and transfer a part of their energy to a cold particles. Similarly the cold air particles descend downwards to fill the space vacated by the hot particles. The circulation pattern upward movement of the warm air and the downward movement of cool air is called the convection current. 24

25 Free Convection: Hot water heating system Water serves as the medium for carrying heat to all rooms of the building Water is heated in a boiler installed at the base of the building. The hot water becomes lighter, rise up in the left hand vertical pipes and passes through the radiators fitted in different rooms of building. The radiators get heated and dissipate heat to the rooms. After losing heat to the radiators, the water gest cooled and returns back to the boiler through the pipe on the right. Convection current build up. 25

26 The situation in Free Convection No bulk flow Flow caused by buoyancy Buoyancy caused by density difference Density difference caused by temperature difference Fluid flow and heat transfer linked to each other. Also called Free Convection 26

27 Advantages/Disadvantages of Free Convection No bulk flow-no power consumption (not consume any power to drive pump or rotate blower or compressor) No noise quiet operation (No fluid flow equipment require) Hardly any vibration (No bulk flow) Heat transfer coefficients are low Large area requirement Orientation dependence (Natural convection depends on density difference and gravity, the direction of gravity and the orientation of surface play an important role) Difficult to control (no fluid flow, no need to switch on blower) 27

28 Types of Convection In forced convection, the fluid velocity is much higher than that of free convection, hence heat exchange rate is more in the case of forced convection. 28

29 Force Convection Single phase forced convection Governing equations Flow through a tube (Internal Flow) Boundary layer over a flat plate (External Flow) Flow across a cylinder (External Flow) Analogy Free Convection Single phase natural convection Some dimensionless numbers Vertical Flat plate Horizontal cylinders Horizontal plates Enclosed spaces-cavities Some correlations 29

30 Newton s law of cooling h convection heat transfer coefficient, W/m 2 C A s the surface area through which convection heat transfer takes place, m 2 T s the surface temperature (solid surface), C T the temperature of the fluid sufficiently far from the surface (fluid free stream), C 30

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32 Convection heat transfer coefficient (h) The convection heat transfer coefficient h is not a property of the fluid. It is an experimentally determined parameter whose value depends on following parameters (i) Fluid flow: Laminar or turbulent, boundary layer configuration etc. (ii) Thermo physical properties of the fluid: such dynamic viscosity μ, thermal conductivity k, density ρ, specific heat c p, fluid velocity V and coefficient of expansion (iii) Surface condition: roughness and cleanliness (iv) Geometry and orientation of the surface: Plate, tube etc. Placed horizontally or vertically 32

33 Laminar and Turbulent flow Laminar flow (Streamline flow): the flow is characterized by smooth streamlines and highly-ordered motion (In this type of flow, the fluid moves in layers, each fluid particle follow a smooth and continuous path. The fluid particles in each layer remain in orderly sequence without mixing with each other). Turbulent flow: the flow is characterized by velocity fluctuations and highly disordered motion (the flow is no more a streamlined flow and eddies of various size are formed in the flow) The transition from laminar to turbulent flow does not occur suddenly. 33

34 Laminar and Turbulent flow Laminar Reynolds showed that the nature of fluid flow is governed by the following parameter: Flow velocity, v Density of fluid, ρ Dynamic viscosity of fluid, µ Diameter D i.e. dimension of flow passage d d Turbulent Nature of flow Range of Re The grouping of above variables results into a dimensionless quantity called the Reynolds number. Laminar Transition Turbulent Re< <Re<4000 Re>4000 Re= v D ρ/ µ 34

35 Boundary Layer The concept of boundary layer was introduced by Prandtl in When a real fluid (viscous fluid) flows past a stationary solid boundary a layer of fluid which comes in contact with the boundary surface adheres to it (on account of viscosity) and condition of no slip occurs (the no slip condition implies that the velocity of fluid at a solid boundary must same as that of boundary itself). Thus the layer of fluid which cannot slip away from the boundary surface undergoes retardation. Variables T Ts x q 35

36 Boundary Layer This retarded layer further causes retardation for the adjacent layers of the fluid, thereby developing a small region in the immediate vicinity of the boundary surface in which the velocity of the flowing fluid increases rapidly from zero at the boundary surface and approaches the velocity of main stream. The layer adjacent to the boundary is known as boundary layer. Boundary layer is formed whenever there is relative motion between the boundary and the fluid. Two types of boundary layer (1) Hydrodynamic boundary layer (2) Thermal boundary layer 36

37 (1) Hydrodynamic boundary layer Boundary Layer The fluid velocity decreases as it approaches the solid surface, reaches to zero in the fluid layer immediately next to the surface. This thin layer of stagnant fluid has been called hydrodynamic boundary layer. Its thickness δ is defined as the distance from the plate surface at which the velocity approach 99% of free stream velocity. (2) Thermal boundary layer A region of fluid motion near the plate which temperature gradient exist is thermal boundary layer and its thickness δ t is defined as the value of y from the plate surface at which t t s s t t

38 Boundary Layer Velocity profile temperature profile Vα δ = Value of y at which V=0.99V α δ t = Value of y at which t t s s t t 0.99 Direction of heat flow 38