The concepts of Thermodynamics are extremely important on the application for thermal systems design. As they present heat transfer process by conduction on the warmth source and sink, evaporation and condensation of the working fluid, the whole cycle that those devices perform are often well explained by the laws of thermodynamics
The Laws of Thermodynamics presented here are seen in every textbook associated with this subject, which are the essential tools for any research and application where heat and mass transfer are involved. Conditions for the equilibrium and energy transfer in processes are discussed as in other books that present this approach on thermodynamics phenomenon, whether or not they are associated with the fundamentals of specific applications.
In the case of the primary Law of Thermodynamics, the processes are treated using an expression that describes the energy conservation. during this case, the energy from a source are often modified and used for an additional destination. this is often the case of energy stored in fuels that’s transformed in energy that runs the internal combustion engines. Another classical example is that the P.E. , where the peak difference on the water reservoir are often transformed in energy which will end in electricity generation. Therefore, the relations between the transformed energy can be well established.
The first law that states that energy cannot be created or destroyed and that it is therefore possible to account for any change in the internal energy of a system ∆E by an exchange of heat (q) and/or work (w) with the surroundings ∆E = Q – w.
Energy Balance for Closed System
A closed system is defined when a specific quantity of matter is under study. A closed system always contains an equivalent matter. There are often no transfer of mass across its boundary. A special sort of closed system that doesn’t interact in any way with its surroundings is named an isolated system .rigid tank and piston cylinder are types of closed system.
The energy balance states that:
The energy balance can be expressed in symbols as an alternative form of the energy balance equation.
∆U is change internal energy, ∆Ek is change in kinetic energy and ∆Ep is change in potential energy, Q is heat transferred to the system and W is work done by the system.
Closed System First Law of a Cycle
The energy balance for any system undergoing a thermodynamic cycle takes the form where Q cycle and W cycle represent net amounts of energy transfer by heat and work, respectively, for the cycle. Since the system is returned to its initial state after the cycle, there’s no net change in its energy. Therefore, the left side of Eq. equals zero, and therefore the equation reduces to The previous Equation is an expression of the conservation of energy principle that has got to be satisfied by every thermodynamic cycle, no matter the sequence of processes followed by the system undergoing the cycle or the character of the substances making up the system.
MASS & ENERGY ANALYSIS OF CONTROL VOLUME
Conservation of Mass
Conservation of mass , principle that the mass of an object or collection of objects never changes, regardless of how the constituent parts rearrange themselves. Mass has been viewed in physics in two compatible ways. On the one hand, it’s seen as a measure of inertia, the opposition that free bodies offer to forces: trucks are harder to maneuver and to prevent than less massive cars. On the opposite hand, mass is seen as giving rise to gravity, which accounts for the load of an object: trucks are heavier than cars. The 2 views of mass are generally considered equivalent. Thus, from the attitude of either mass or mass , consistent with the principle of mass conservation, different measurements of the mass of an object taken under various circumstances should be an equivalent.
Mass and volume Flow Rates
Mass flow is that the rate of movement of a huge fluid through a unit area. Mass flow depends on the density, velocity of the fluid and therefore the area of the cross section. Meaning, it’s the movement of mass per unit time. It’s units are kg/s.
Principal of Conservation of Mass
the conservation of mass principle states that denoting the mass contained within the control volume at time t by mcv(t), this statement of the conservation of mass principle are often expressed in symbols as where dmcv/dt is the time rate of change of mass within the control volume, and mi and me are the instantaneous mass flow rates at the inlet and exit, respectively. As for the symbols W and Q, the dots in the quantities mi and me denote time rates of transfer. In SI, all terms in Eq. are expressed in kg/s. In English units, they are expressed in lb/s. For a discussion of the development of Eq., see the box.In general, there may be several locations on the boundary through which mass enters or exits. This can be accounted for by summing, as follows.
The previous Equation is that the mass rate balance for control volumes with several inlets and exits. It is a sort of the conservation of mass principle commonly employed in engineering.
Steady-State Form of the Mass Rate Balance
Many engineering systems are often idealized as being at steady state, meaning that each one properties are unchanging in time. For an impact volume at steady state, the identity of the matter within the control volume changes continuously, but the entire amount present at any instant remains constant.
That is, the entire incoming and outgoing rates of mass flow are equal. Note that equality of total incoming and outgoing rates of mass flow doesn’t necessarily imply that an impact volume is at steady state. Although the entire amount of mass within the control volume at any instant would be constant, other properties such as temperature and pressure could be varying with time. When a control volume is at steady state, every property is independent of your time . Also note that the steady state assumption and therefore the one-dimensional flow assumption are independent idealizations .One doesn’t imply the opposite.
Steady-flow Engineering Devices
1. Nozzles and Diffusers
A nozzle may be a flow passage of varying cross-sectional area during which the speed of a gas or liquid increases within the direction of flow. In a diffuser the gas or liquid deceler-ates within the direction of flow. Figure shows a nozzle during which the cross-sectional area decreases within the direction of flow and a diffuser during which the walls of the flow passage diverge. Observe that as velocity increases pressure decreases, and conversely.
For many readers the foremost familiar application of a nozzle is its use with a gardenhose. But nozzles and diffusers have several important engineering applications. a nozzle and diffuser are combined during a wind-tunnel test facility. Ducts with converging and diverging passages are commonly utilized in distributing cool and warmair in building air-conditioning systems. Nozzles and diffusers are also key components of turbojet engines.
Nozzle and Diffuser Modeling Considerations
For a control volume enclosing a nozzle or diffuser, the sole work is flow work at locations where mass enters and exits the control volume, therefore the term Wcv drops out of the energy rate balance. The change in P.E. from inlet to exit is neg-ligible under most conditions. Thus, the underlined terms of Eq (repeated below) drop out, leaving the enthalpy, K.E. , and warmth transfer terms, as shown by Eq.
Where is that the mass flow . The term Qcv representing heat transfer with the surroundings normally would be unavoidable (or stray) heat transfer, and this is often often small enough relative to the enthalpy and K.E. terms that it can also be neglected, giving simply.
A turbine may be a device during which power is developed as a results of a gas or liquid passing through a group of blades attached to a shaft liberal to rotate. A schematic of an axial-flow steam or turbine . Such turbines are widely used for power generation in vapor power plants, turbine power plants, and aircraft engines . In these applications, superheated steam or a gas enters the turbine and expands to a lower pressure as power is generated. Ahydraulic turbine coupled to a generator installed during a dam . As water flows from higher to lower elevation through the turbine, the turbine provides shaft power to the generator. The generator converts shaft power to electricity. this sort of generation is named hydropower.
Steam and Gas Turbine Modeling Considerations
With a proper selection of the control volume enclosing a steam or turbine , the net K.E. of the matter flowing across the boundary is typically small enough to be neglected. internet P.E. of the flowing matter is also typically negligible. Thus, the underlined terms of Eq. (repeated below) drop out, leaving the power, enthalpy, and heat transfer terms, as shown by Eq.
where is that the mass flow . the sole heat transfer between the turbine and surroundings normally would be unavoidable (or stray) heat transfer, and this is often often small enough relative to the facility and enthalpy terms that it can also be neglected, giving simply.
3. Compressors and Pumps
Compressors and pumps are devices during which work is done on the substance flowing through them so as to vary the state of the substance, typically to extend the pressure and/or elevation. The term compressor is employed when the substance may be a gas (vapor) and therefore the term pump is employed when the substance may be a liquid.
Compressor and Pump Modeling Considerations for a control volume enclosing a compressor, the mass and energy rate balances reduce at steady state as for the case of turbines.
Heat transfer with the surroundings is usually a secondary effect which will be neglected, giving as for turbines.
For pumps, heat transfer is usually a secondary effect. but the kinetic and potential energy could also be significant counting on the appliance . Be sure to note that for compressors and pumps, the value of is negative because an influence input is required.
4. Heat Exchangers
Heat exchangers have innumerable domestic and industrial applications, including use in home heating and cooling systems, automotive systems, electric power generation ,and chemical processing.
One common sort of device may be a mixing chamber during which hot and coldstreams are mixed directly. The open feed water heater, which is a component of the vapor power systems , is an example of th is sort of device.
Another common sort of exchanger is one during which a gas or liquid is separated from another gas or liquid by a wall through which energy is conducted. These heat exchangers, known as recuperators , take many various forms. Counter flow and parallel tube.
Heat Exchanger Modeling Considerations
heat exchangers can involve multiple inlets and exits. For a control volume enclosing a device, the sole work is flow work on the places where matter enters and exits, therefore the term drops out of the energy rate balance .In addition, the kinetic and potential energies of the flowing streams usually can be ignored at the inlets and exits.
Although high rates of energy transfer within the heat exchanger occur, heat transfer with the surroundings is usually sufficiently small to be neglected. Thus, the term of Eq. would drop out, leaving just the enthalpy terms.
5. Throttling Devices
A significant reduction in pressure are often achieved just by introducing a restriction into a line through which a gas or liquid flows. this is often commonly done by means of a partially opened valve or a porous plug.
An application of throttling occurs in vapor-compression refrigeration systems, where a valve is employed to scale back the pressure of the refrigerant from the pressure at the exit of the condenser to the lower pressure existing within the evaporato.
Throttling Device Modeling Considerations
For a control volume enclosing a throttling device, the sole work is flow work on locations where mass enters and exits the control volume, therefore the term drops out of the energy rate balance. there’s usually no significant heat transfer with the environment ,and the change in P.E. from inlet to exit is negligible. Thus,the underlined terms of Eq. (repeated below) drop out, leaving the enthalpy and K.E. terms, as shown by Eq. That is,
6. Mixing Chambers
In engineering applications, the steady-flow mixing of two streams of an equivalent fluid is another common process. The section where the mixing process takes place is mentioned as a mixing chamber. An open feedwater heater is an example of mixing chamber. If 1 and 2 denote the inlets and three denotes the exit a mass balance gives.
Mixing chambers are usually well insulated. Although the temperatures of the flow streams could also be quite different from the temperature of the environment, the foremost important energy transfer is between the 2 fluids and not between the fluids and therefore the environment the energy balance for mixing chamber is.