Fluid flow enthalpy

Most physical phenomena in fires can be described as the transfer of heat and mass and chemical reactions in either gas or condensed phases. The physics of the fluid flow are governed by the conservation equations of mass, momentum, and enthalpy ... 

In the mixing length theory it is assumed basically that lumps of fluid are carried transversely across the fluid flow by the turbulent eddies and during this motion they preserve their initial momentum and enthalpy. The motion continues over a transverse distance, lm, after which the "lumps interact with other fluid "lumps ... 

For an isothermal fluid flow described by the Redlich-Kwong equation of state, develop expressions in terms of the initial temperature and the initial and final volumes for the changes in internal energy, enthalpy, entropy, and the Gibbs free energy. 

Sometimes, fluids flow through a restriction, such as an orifice, a valve, or a porous medium, and a pressure drop occurs adiabatically. If the changes in kinetic and potential energies are negligible, the flow process is a throtding process, which causes no change in enthalpy at the inlet and outlet, and we have AH = 0. Some properties of throtding processes are ... 

In the analysis of systems that involve fluid flow, wc frequently encounter the combination of properties u and Pv. For the sake of simplicity and convenience, this combination is defined as enthalpy /t. That is, It u + Pv where the term Pv represents the. flow energy of the fluid (also called the flow work), which is the energy needed to push a fluid and to maintain flow. In the energy analysis of flowing fluids, it is convenient to treat the flow energy as part of the energy of the fluid and to represent the microscopic energy of a fluid stream by enthalpy h (Fig. 1 7). 

The internal energy ti represents the microscopic energy of a nonflowing fluid, whereas enthalpy h represents the microscopic energy of a flowing fluid. 

Throttling Process Fluid flowing through a restriction, such as an orifice, without appreciable change in kinetic or potential energy undergoes a finite pressure drop. This throttling process produces no shaft work, and in the absence of heat transfer, Eq. (4-155) reduces to AH = 0 or Ha = Hi. The process therefore occurs at constant enthalpy. 

The velocity held is determined by the characteristic length L0, and velocity w0 e.g. the entry velocity in a tube or the undisturbed velocity of a fluid flowing around a body, along with the density g and viscosity rj of the fluid. While density already plays a role in frictionless flow, the viscosity is the fluid property which is characteristic in friction flow and in the development of the boundary layer. The two material properties, thermal conductivity A and specific heat capacity c, of the fluid are important for the determination of the temperature held in conjunction with the characteristic temperature difference Ai 0. The specihc heat capacity links the enthalpy of the fluid to its temperature. 

The two fluids flow through the heat exchanger without undergoing a phase change, i.e. they do not boil or condense. The small change in specific enthalpy with pressure is neglected. Therefore only the temperature dependence is important, and with... 

THE ASTERISK CONDITION. The state of the fluid moving at its acoustic velocity is important in some processes of compressible-fluid flow, The condition where u = a and = 1 is called the asterisk condition, and the pressure, temperature, density, and enthalpy arc denoted by p, T, p, and H at this state. 

For surfaces in unbounded convection, such as plates, tubes, bodies of revolution, etc., immersed in a large body of fluid, it is customary to define h in Eq. (1.12) with 7 as the temperature of the fluid far away from the surface, often identified as T. (Fig. 1.2). For bounded convection, including such cases as fluids flowing in tubes or channels, across tubes in bundles, etc., Tf is usually taken as the enthalpy-mixed-mean temperature, customarily identified as T . 

Modem adiabatic calorimeters employ a technique whereby the enthalpy of vaporization is measured under conditions in which a measured amount of electrical energy is supplied to a heater immersed in the sample to compensate for the heat absorbed by the substance during the evaporation and hence the temperature is kept constant. The main differences among adiabatic calorimeters are that the vapour flows out of the calorimeter at atmospheric pressure (those of Mathews and Fehlandt [65]), into a vacuum, [67,69-71] into a gas stream [68], or into a closed recirculation system with continuous fluid flow [66]. 

The necessary enthalpy increase for the fluid flowing through the liquid-type collector is... 

EXAMPLE 2.7-3. Energy Balance in Flow Calorimeter A flow calorimeter is being used to measure the enthalpy of steam. The calorimeter, which is a horizontal insulated pipe, consists of an electric heater immersed in a fluid flowing at steady state. Liquid water at 0°C at a rate of 0.3964 kg/min enters the calorimeter at point 1. The liquid is vaporized completely by the heater, where 19.63 kW is added and steam leaves point 2 at 250°C and 150 kPa absolute. Calculate the exit enthalpy of the steam if the liquid enthalpy at 0°C is set arbitrarily as 0. The... 

If there is no heat transfer to/from the tank Q = 0), then steady-state fluid temperature inside the tank is same as that of the inlet temperature. Thus, the inlet mass flow rate has no bearing on the temperature of the stored fluid as long as there is no heat transfer from the stored fluid it is only the inlet fluid temperature which influences the temperature of the stored fluid. Even if there is some kind of heat transfer which is proportional to temperature of the fluid with a constant coefficient of proportionality, but not related to the quantity of the stored mass (the stored mass is anyway constant in steady state), the heat transfer does not influence the steady-state temperature of the stored mass. The later is similar to the enthalpy flow due to fluid flow out of the tank. 

Reducing the pressure of the working fluid. The fluid flows through an expansion valve where its pressure is reduced without much enthalpy change. The liquid is again at low pressure, poised for vaporization, and the cycle repeats. This is how vaporization cycles can be used to pump heat from cold places to hot places. 

For isentropic flow, the energy equation can be written as follows, noting that the addition of internal and flow energies can be written as the enthalpy (h) of the fluid ... 

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