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Answer: Nusselt number (Nu) is defined as the ratio of conductive heat transfer resistance to convective heat transfer resistance. Higher the value, lesser is the convective heat transfer resistance and easier is the heat transfer from fluid to solid wall or vice versa. Heat transfer coefficient, which is used for the design of heat exchanger for a system, is obtained from the values of Nu which is again obtained from suitable correlations/experimentations.
Answer: Its is difficult. Please refer to the Test/References as mentioned for any difficulties.
Answer: Your thought is good but one-sided. You may like to know that kachha houses are better because of that reason only. The porous walls do contain air which acts as insulator to heat. Thus, it is comfortable for both in winter and summer. You are thinking only the heat transfer part. Possibly the more important part is the stability of the structure and the corresponding cost. One has to be careful about the stability of the hollow walls.
Answer: Please don't cofuse yourself. As you rightly said, conduction area is perpendicular to the direction of heat flow. In case of cylindrical fins, the temperature in a cross-section is assumed to be same within the fin. so heat transfer by conduction is only along the length of fin where the temperature difference is existing. Heat transfer from the circumference to the surrounding is taken as by convection. The above assumption is realistic in the sense that metallic fins have very high conductivity and they are usually very thin. If you take the variation of temperature along radial direction in a cross-section, then it will be a case of two-dimensional heat transfer problem, which is normally avoided if not so necessary.
Answer: Sorry for late replying. The question is for very specific applications and hence beyond the scope of this course. However, I should suggest you to approach to some experts in solar thermal etc for your queries.
Answer: It may be noted that Fouling factor for a perfectly clean surface is zero. It is, for a fluid, obtained by doing experimentation of heat transfer with fresh surface and surface after fouled by the fluid. Fouling coefficients for various types of fluids can be seen from various books. Every fluid is going to foul a surface and therefore the fouling coefficient is taken into consideration while designing a heat exchanger.
Answer: It is not wise to generalize the statement like this way. Condensation and boiling can take place in various forms like film type and drop wise condensation and nucleate boiling, film boiling, pool boiling, forced boiling etc. If compared between film condensation and film boiling, lower thermal conductivity of vapour than liquid makes the condensation heat transfer coefficient higher than boiling.
Answer: Fourer’s first law of heat conduction is valid irrespective of the geometry. Area is to be chosen accordingly. Area can be a variable with the coordinate. For irregular shaped objects, it tried to approximate as regular shaped one if lies within acceptable error. Otherwise, fine meshing of the solid is needed and conduction equation is applied accordingly. These are complicated cases and are not normally covered in UG level. However, courses like computational transport phenomena may cover such situation.
Answer: Please note that every mathematical correlation is associated with certain limitation such as range of the pertinent variables (such as Reynold number, Prandtle number, Nusselt number, etc., type of fluid to be handled, nature of process involved, the type of equipment/ instrument used, etc. So, you will have to first identify these variables/parameters and then accordingly you may short-list a few correlations. Now you may compare between the percentage improvement of the result versus the complations involved while using the complicated correlation over a simpler one. Then a judical decision is to be taken.
Answer: Please note that there is not hard and fast rule for such selection. However, I would like to give you some guidelines for the same. 1. See that all the core variables together include all the primary dimensions 2. Do not include two core variables having similar dimensions or power of similar dimensions (like L and L cube, etc.) 3. The variables can be classified as Operating variables, Geometric variables, and physical properties. Try to include atleast one from each of the three classes. 4. With properly identified core variables, one will get the familiar dimensionless groups other wise the dimensionless group obtained might be combination of several familiar dimensionless groups.
Answer: Heater is a device that heat or provides heat. Furnace is a specific type of heater. It is an enclosed chamber that is used for heating. Sometimes fuel is converted to heat in a furnace. Temperature of a furnace is usually very high and it can be of various shape and sizes. Low temperature enclosed equipment for heating is called oven. All ovens and Furnaces are heaters but all heaters are not oven or furnaces.
Answer: Dirt has higher conductive heat transfer resistnace than clean meatllic surfaces. Normal surfaces do contain some amount dirt on it. The lower heat transfer resistance of clean surface is reflected through higher value of h in case of condensation heat transfer over clean surface.
Answer: At the start of the coourse I had suggested a few books, which may looked into for the problems you asked for.
Answer: Sorry for replying late. Along the height of the wall, the flow situation varies and thus the mechanism of heat transfer. As you understand, heat trasnfer coefficient is dependent on Reynold number and Prandtl number which are again depenent of flow dynamics and diffisivity mechanisms etc. Therefore, heat transfer coefficient is function may vary along wall height where variation of temperature induces varying flow dynamics and diffusion mechanism.
