Question I [Total: 20 marks] (a) Explain the definitions of \"closed system\" an

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Question I [Total: 20 marks] (a) Explain the definitions of "closed system" and "open system" (b) Consider a refrigerator running in a well-insulated room (all the doors and windows [2 marks] are tightly closed and no heat transfer through the walls and the windows). How will the temperature in the room change if the door of the refrigerator is left open over a long period of time? Why? [2 marks 12 marks] (c) What are the three mechanisms of heat transfer? (d) Does the temperature increase or decrease as the flow passes an insulated nozzle? (e) When is the flow through a control volume steady? (f) Identify the differences between heat pumps and refrigerators Why? [2 marks] [2 marks [2 marks] [2 marks] (2) An inventor proposes that, to increase the efficiency of a heat engine, to remove the heat rejection to the low temperature sink, forcing all heat to convert to work. Is that [2 marks] (h) Which state of matter (among solid, liquid and gas) has the highest entropy and which [2 marks] ) When a refrigerator operates, it cools down the refrigerated space. This implies that entropy of the refrigerated space decreases, so entropy change is negative. But it is known that entropy change in an isolated system should be equal or greater than zero. possible? Explain. the lowest? Where is the problem in the previous statements? [2 marks] O In an adiabatic turbine, which expansion produces the most power? The isentropic [2 marks] or the real expansion?

Explanation / Answer

a) Ludwig Bertalanffy describes two types of systems: open systems and closed systems. The open systems are systems that allow interactions between their internal elements and the environment. An open system is defined as a “system in exchange of matter with its environment, presenting import and export, building-up and breaking-down of its material components.” Closed systems, on the other hand, are held to be isolated from their environment. Equilibrium thermodynamics, for example, is a field of study that applies to closed systems.

b) The refrigerated space is at a lesser temperature than the surroundings. Which means that when you open the door, the heat gets absorbed into the refrigerator (that’s by one of the founding principles of heat transfer- the thermal gradient) and the temperature of the refrigerated space increases.
As the heat is transferred from the room, the room temperature decreases.
However, this is under assumption that heat generated by the refrigerator is dissipated outside the room. For same room, the temperature more likely will increase because the back of your fridge is much hotter and more than compensates any temperature drop by opening of fridge door.

c)There are three mechanisms by which heat (energy) is transferred in the atmosphere: Radiation, Conduction, Convection.i) Conduction, or the transfer of energy from matter to adjacent matter by direct contact, without intermixing or flow of any material.ii) Convection, or the transfer of energy by the bulk mixing of clumps of material. In natural convection it is the difference in density of hot and cold fluid which causes the mixing. In forced convection a mechanical agitator or an externally imposed pressure difference (by fan or compressor) causes the mixing.iii) Radiation such as light, infrared, ultraviolet and radio waves which emanate from a hot body and are absorbed by a cooler body

d) When a gas passes through a nozzle, its velocity increases on the expense ofdecrease of its pressure. Now temperature change is directly proportional to pressure change. So its temperature decreases due to decrease of its pressure. ... But the pressure of gas decreases during expansion so the temperature decreases.

e) if the properties in a control volume involve no change with time at any fixed position.

f) Following are the main difference between Heat Pump and Refrigerator.i) A heat pump is similar to a refrigerator. The only point of difference between the two is of the operating temperatures. The working temperatures in a refrigerator are of the colder level and atmosphere, whereas working temperatures in heat pump are of hotter level and atmosphere.ii) A refrigerator is a reversed heat engine. Heat is transferred from the lower temperature level to higher temperature by applying external work to maintain the temperature below atmospheric temperature.

g) In thermodynamics, heat engines are often modeled using a standard engineering model such as the Otto cycle. The theoretical model can be refined and augmented with actual data from an operating engine, using tools such as an indicator diagram. Since very few actual implementations of heat engines exactly match their underlying thermodynamic cycles, one could say that a thermodynamic cycle is an ideal case of a mechanical engine. In any case, fully understanding an engine and its efficiency requires gaining a good understanding of the (possibly simplified or idealized) theoretical model, the practical nuances of an actual mechanical engine, and the discrepancies between the two

The efficiency of various heat engines proposed or used today has a large range:

h) If we look at the three states of matter: Solid, Liquid and Gas, we can see that the gas particles move freely and therefore, the degree of randomness is the highest.

For liquid state, still the particles are moving but less freely than the gas particles and more than the solid particles.

Therefore, Sgas>>Sliquid>Ssolid

j)For an adiabatic turbinewhich undergoes a steady-flow process, its inlet and exit pressures are fixed. Hence, the idealized process forturbine is an isentropic process between the inlet and exit pressures. The desired output from a turbine is the work output.Adiabatic heating and cooling. The adiabatic compression of a gas causes a rise in temperature of the gas. Adiabatic expansion against pressure, or a spring, causes a drop in temperature. In contrast, free expansion is an isothermal process for an ideal gas

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