Natural convection, heat-transfer coefficients

During a rotational moulding operation an aluminium mould with a uniform thickness of 3 mm is put into an oven at 300°C. If the initial temperature of the mould is 23°C, estimate the time taken for it to reach 250°C. The natural convection heat transfer coefficient is 28.4 J/m s. 

Da.skalaki E. Natural convection heat transfer coefficients from vertical and horizontal surfaces for building applications. Energy and Buddings, vol. 20, no.. T, 1994. 

The heat loss through a firebrick furnace wall 0.2 in thick is to be reduced by addition of a layer of insulating brick to the outside. What is the thickness of insulating brick necessary to reduce the heat loss to 400 W/m2 The inside furnace wall temperature, is 1573 K. the ambient air adjacent to the furnace exterior is at 293 K and the natural convection heat transfer coefficient at the exterior surface is given by h S.OAT11 23 W/in2 K, where AT is the temperature difference between the surface and the ambient air,... 

Natural-convection heat-transfer coefficients may be calculated using the equations... 

Related Calculations. The minimum temperature difference required for nucleate boihng to occur can also be determined by plotting the equations for nucleate-boihng and natural-convection heat-transfer coefficients. The intersection of these two lines represents the required temperature difference... 

Gases are nearly transparent to radiatioo, and thus heat transfer through a gas layer is by simultaneous convection (or conduction, if the gas is quiescent) and radiation. Natural convection heat transfer coefficients are typically very low compared to those for forced convection. Therefore, radiation is usually disregarded in forced convection problems, but it must be considered in natural convection problems that involve a gas. This is especially the case for surfaces with high emissivities. For example, about half of the heat transfer through the air. space of a double-pane window is by radiation, The total rate of heat transfer is determined by adding the convection and radiation components,... 

Natural convection heat transfer occurs when a solid surface is in contact with a gas or liquid which is at a different temperature from the surface. Density differences in the ffuid arising from the heating process provide the buoyancy force required to move the ffuid. Free or natural convection is observed as a result of the motion of the fluid. An example of heat transfer by natural convection is a hot radiator used for heating a room. Cold air encountering the radiator is heated and rises in natural convection because of buoyancy forces. The theoretical derivation of equations for natural convection heat-transfer coefficients requires the solution of motion and energy equations. 

Natural convection from vertical planes and cylinders. For an isothermal vertical surface or plate with height L less than 1 m (P3), the average natural convection heat-transfer coefficient can be expressed by the following general equation ... 

Natural convection heat transfer coefficient for the outside of a pipe... 

The kind of convective heat transfer—forced convection or natural (at floor, wall, or ceiling)—must be considered and taken into account by selecting appropriate values for the convective heat transfer coefficient see Eq. (11.14)). Thus, the heat transfer coefficient implicitly assumes the flow situation at the surface. Normally, coefficients for convective heat transfer are considered as a preset constant parameter (the coefficient may be defined as variable, however, depending on other parameters). Therefore, the selection of appropriate values is crucial. Values for heat transfer coefficients can be found in several references a comprehensive summary is given in Daskalaki. ... 

Let us examine methanol. Its flashpoint temperature is 12 to 16 °C (285-289 K) or, say, 15 °C. If this is in an open cup, then the concentration near the surface is Xl = 6.7 %. Performed under normal room temperatures of, say, 25 °C, the temperature profile would be as in Figure 6.2. This must be the case because heat must be added from the air to cause this evaporated fuel vapor at the surface. This decrease in temperature of an evaporating surface below its environment is sometimes referred to as evaporative cooling. If the convective heat transfer coefficient, typical of natural convection, is,... 

Example 6.3 Consider Example 6.2 for a shallow pool of methanol with its bottom surface maintained at 25 °C. Assume that natural convection occurs in the liquid with an effective convective heat transfer coefficient in the liquid taken as 10 W/m2 K. Find the surface temperature, surface vapor mass fraction and the evaporation flux for this pool. 

Conduction is treated from both the analytical and the numerical viewpoint, so that the reader is afforded the insight which is gained from analytical solutions as well as the important tools of numerical analysis which must often be used in practice. A similar procedure is followed in the presentation of convection heat transfer. An integral analysis of both free- and forced-convection boundary layers is used to present a physical picture of the convection process. From this physical description inferences may be drawn which naturally lead to the presentation of empirical and practical relations for calculating convection heat-transfer coefficients. Because it provides an easier instruction vehicle than other methods, the radiation-network method is used extensively in the introduction of analysis of radiation systems, while a more generalized formulation is given later. 

There are upper and lower limits of applicability of the equation above. The lower limit results because natural-convection heat transfer governs at low temperature differences between the surface and the fluid. The upper limit results because a transition to film boiling occurs at high temperature differences. In film boiling, a layer of vapor blankets the heat-transfer surface and no liquid reaches the surface. Heat transfer occurs as a result of conduction across the vapor film as well as by radiation. Film-boiling heat-transfer coefficients are much less than those for nucleate boiling. For further discussion of boiling heat transfer, see Refs. 5 and 6. 

The convection heat transfer coefficient /i is not a property of the fluid. It is an experimentally determined parameter whose value depends on all the variables influencing convection such as the surface geometry, the nature of fluid motion, Ihe properties of the fluid, and the bulk fluid velocity. Typical values of h arc given in Table 1-5. 

In a conventional oven, the air in the oven is first heated to the desired temperature by the electric or gas heating element. This preheating may take several minutes. The heat is then transferred from the air to the skin of the chicken by natural convection in older ovens or by forced convection in the nevrer convection ovens that utilize a fan. The air motion in convection ovens increases the convection heat transfer coefficient and thus decreases the cooking time. Heat is then conducted toward the inner parts of the chicken from its outer parts as in microwave ovens. 

A 3-m internal diameter spherical tank made of 2-cm-thlck stainless steel (k = 15 W/m O is used to store iced water at r i == 0°C. Ttie tank is located in a room whose temperature is 7 j - 22°C. The walis of the room are also at 22°C. The outer surface of the tank is black and heat transfer between the outer surface of the tank and the surroundings is by natural convection and radiation. The convection heat transfer coefficients at the inner and the outer surfaces of the tank are h, = 80 W/m °C and I12 = 10 W/m °C, respectively. Determine (a) the rate of heat transfer to the iced water in the lank and (b) the amount of ice at 0 C that melts during a 2d-h period. 

The use of fins is most effective in applications involving a low convection heat transfer coefficient. Thus, the use of fins is more easily justified when the medium is a gas instead of a liquid and the heat transfer is by natural convection instead of by forced convection. Therefore, it is no coincidence that in liquid-to-gas heat exchangers such as the car radiator, fins are placed on the gas side. 

C In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural convection or forced convection Why ... 

The convection beat transfer coefficient is a strong function of velocity the higher the velocity, the higher die convection heat transfer coefficient. The fluid velocities associated with natural convection are low, typically less than 1 nVs. Therefore, the heat transfer coefficients encountered iti natural convection are usually much lower than those encountered in forced convection. Yet several types of heat transfer equipment are designed to operate under natural convection conditions instead of forced convection, because natural convection does not require the use of a fluid mover. 

For a given fluid, it is observed that the parameter Gi/Re represents the importance of natural convection relative to forced convection. This is not surprising since the convection heat transfer coefficient is a strong function of the Reynolds number Re in forced convection and the Grashof number Gr in natural convection. 

Consider a l.2-m-high and 2-m-wide glass window with a thickness of 6 nun, thermal conductivity k = 0.78 W/m C, and emissivity e = 0.9. The room and the walls that face the window are maintained at 25°C, and the average temperature of the inner surface of the window is measured to be 5°C. If the temperature of Ihe outdoors is -5 C, determine (a) the convection heat transfer coefficient on Ihe inner surface of the window, (b) the rate of total heat transfer through the window, and (c) the combined natural convection and radiation beat transfer coefficient on the outer... 

Heat exchangers are complicated devices, and the results obtained with the simplifled approaches presented above should be used with care. For example, we assumed that the overall heat transfer coefficient V is constant throughout the heat exchanger and tliat the convection heat transfer coefficients can he predicted using the convection correlations. However, it should be kept in mind that the uncertainty in the predicted value of U can exceed 30 percent. Thus, it is natural to tend to overdesign the hear exchangers in order to avoid unpleasant surprises. 

To determine the Rayleigh number, we need to know the surface temperature of the glass, v/hich is not available. Therefore, it is clear that the solution require a trial-and-error approach unless we use an equation solver such as EES. Assurtiing the glass cover temperature to be 40 C, the Rayleigh number, the Nusselt number, the convection heat transfer coefficient, and the rate of natural convection heat transfer from the glass cover to the ambient air are determined to be... 

Recognizing that this is a natural convection problem with hot horizontal surface facing up, the Nussell number and the convection heat transfer coefficients are determined to be... 

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