Thermal hydraulics (also called thermohydraulics) is the study of hydraulic flow in thermal fluids. The area can be mainly divided into three parts:

thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...

fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids (liquids, gases, and plasma (physics), plasmas) and the forces on them. Originally applied to water (hydromechanics), it found applications in a wide range of discipl ...

, and

heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

, but they are often closely linked to each other. A common example is

steam Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

generation in

power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an electr ...

s and the associated energy transfer to mechanical motion and the change of states of the

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

while undergoing this process. Thermal-hydraulics analysis can determine important parameters for reactor design such as plant efficiency and coolability of the system. The common adjectives are "thermohydraulic", "thermal-hydraulics" and "thermalhydraulics".

Thermodynamic analysis

In the thermodynamic analysis, all states defined in the system are assumed to be in thermodynamic equilibrium; each state has mechanical, thermal, and phase equilibrium, and there is no macroscopic change with respect to time. For the analysis of the system, the first law and second law of thermodynamics can be applied. In

analysis, a series of states can comprise a cycle. In this case, each state represents condition at the inlet/outlet of individual component. The example of components are

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

compressor,

turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced can be used for generating electrical ...

, reactor, and heat exchanger. By considering the

constitutive equation In physics and engineering, a constitutive equation or constitutive relation is a relation between two or more physical quantities (especially kinetic quantities as related to kinematic quantities) that is specific to a material or substance o ...

for the given type of fluid, thermodynamic state of each point can be analyzed. As a result, the

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For ...

of the cycle can be defined. Examples of the cycle include the Carnot cycle, Brayton cycle, and Rankine cycle. Based on the simple cycle, modified or combined cycle also exists.

Thermo-Hydraulic Improvement Parameter (THIP)

Authors observed that Thermo-hydraulic Parameter (THP) is less sensitive towards the Friction Factor Improvement Factor (FFER). The deviation between the terms (fR/fS) and (fR/fS)0.33 has been found 48 % to 64 % for the range of roughness and other parameters with (Re) 2900 – 14,000, which has been used for the present study. Therefore, to evaluate in equal proportions of enhancement in heat transfer (Nu) and friction factor (f) in the thermal systems a new parameter has been proposed and introduced by present article first author, which is more realistic and it is named as Thermo-hydraulic Improvement Parameter (THIP), and it can be evaluated as the ratio of (NNIF) to (FFIF) ahu et al. Where (NNIF)=Nusselt Number Improvement Factor and (FFIF)=Friction Factor Improvement Factor

Temperature distribution

Temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

is an important quantity to know for the understanding of the system. Material properties such as

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

, and specific heat depend on temperature, and very high or low temperature can bring unexpected changes in the system. In solid, the heat equation can be used to obtain the temperature distribution inside the material with given geometries. For steady-state and static case, the heat equation can be written as :

0 =\,\nabla\cdot k \,\nabla T\ \ +q

where Fourier’s law of conduction is applied. Applying boundary conditions gives a solution for the temperature distribution.

Single-phase heat transfer

In single-phase heat transfer,

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

is often the dominant mechanism of heat transfer. For adiabatic flow where the flow receives heat, the temperature of the coolant changes as it flows. An example of single-phase heat transfer is a gas-cooled reactor and molten-salt reactor. The most convenient way for characterizing the single-phase heat transfer is based on an empirical approach, where the temperature difference between the wall and bulk flow can be obtained from the heat transfer coefficient. The heat transfer coefficient depends on several factors: mode of heat transfer (e.g., internal or external flow), type of fluid, geometry of the system, flow regime (e.g., laminar or

turbulent flow In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by Chaos theory, chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disrupt ...

), boundary condition, etc. Examples of heat transfer correlations are Dittus-Boelter correlation (turbulent forced convection), Churchill & Chu (

natural convection Convection is single or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...

Multi-phase heat transfer

Compared with single-phase heat transfer, heat transfer with a phase change is an effective way of heat transfer. It generally has high value of heat transfer coefficient due to the large value of latent heat of phase change followed by induced mixing of the flow. Boiling and condensation heat transfers are concerned with wide range of phenomena.

Pool boiling

Pool boiling is boiling at a stagnant fluid. Its behavior is well characterized by Nukiyama boiling curve, which shows the relation between the amount of surface superheat and applied heat flux on the surface. With the varying degrees of the superheat, the curve is composed of natural convection, onset of nucleate boiling, nucleate boiling, critical heat flux, transition boiling, and film boiling. Each regime has a different mechanism of heat transfer and has different correlation for heat transfer coefficient.

Flow boiling

Flow boiling is boiling at a flowing fluid. Compared with pool boiling, flow boiling heat transfer depends on many factors including flow pressure, mass flow rate, fluid type, upstream condition, wall materials, system geometry, and applied heat flux. Characterization of flow boiling requires comprehensive consideration of operating condition. In 2021 a prototype

electric vehicle An electric vehicle (EV) is a motor vehicle whose propulsion is powered fully or mostly by electricity. EVs encompass a wide range of transportation modes, including road vehicle, road and rail vehicles, electric boats and Submersible, submer ...

charging cable using flow boiling was able to remove 24.22 kW of heat, allowing the charging current to reach 2,400 amps, far higher than state of the art charging cables that top out at 520 amps.

Critical Heat Flux

Heat transfer coefficient due to nucleate boiling increases with wall superheat until they reach a certain point. When the applied heat flux exceeds the certain limit, heat transfer capability of the flow decreases or significantly drops. Normally, the critical heat flux (CHF) corresponds to departure from nucleate boiling (DNB) in pressurized water reactor (PWR) and dryout in boiling water reactor (BWR). The reduced heat transfer coefficient seen in post-DNB or post-dryout is likely to result in damaging of the boiling surface. Understanding of the exact point and triggering mechanism related to critical heat flux is a topic of interest.

Post-CHF Heat transfer

For DNB type of boiling crisis, the flow is characterized by creeping vapor fluid between liquid and the wall. On top of the convective heat transfer, radiation heat transfer contributes to the heat transfer. After the dryout, the flow regime is shifted from an inverted annular to mist flow.

Other phenomena

Other thermal hydraulics phenomena are subject of interest: * Critical discharge * Countercurrent flow limitation * Condensation * Flow instability * Rewetting

References

{{DEFAULTSORT:Thermal Hydraulics Thermodynamics Fluid dynamics