Analysis of Performance of Cavitation Models with Analytically Calculated Coefficients

Cavitation is often simulated using a mixture model, which considers the transport of an active scalar, namely the vapor fraction αv. Source and sink terms of the transport equation of αv, namely vaporization and condensation terms, rule the dynamics of the cavity and are described through different models. These models contain empirical coefficients generally calibrated through optimization processes. The purpose of this paper is to propose an analytical approach for the calculation of the coefficients, based on the time scales of vaporization and condensation processes. Four different models are compared considering as a test-case a two-dimensional flow around a cylinder. Some relevant quantities are analyzed both for standard value of coefficients, as found in the literature, and the coefficients calculated through the analytical approach. The study shows that the analytical computation of the coefficients of the model substantially improve the results, and the models considered give similar results, both in terms of cavitation regime and mean vapor fraction produced.

Polytropic Change of State Calculations With Condensation or Evaporation and its Use for Thermal Power Cycles

In the paper [1] three polytropic change of state calculation methods for air gases and CO2 are compared. The conclusions are that the most used formulas can produce with the same input data deviations in terms of polytropic efficiency up to 1.5% points even for nearly ideal gases like nitrogen. The paper suggests therefore a recursive algorithm, which is based directly on the classic definition of a polytropic change of state. This definition assumes a constant dissipation rate during an adiabatic change of state. The proposed algorithm can be applied for any gas with an equation of state, which is an unambiguous function of the two variables pressure p and temperature T. The latter condition is not fulfilled by steam in the wetness range because the specific volume v depends not only from p and T but also from the vapor fraction x. The aim of this paper is the development of an analogous recursive “constant dissipation rate algorithm” for a two phase mixture assuming equilibrium conditions between the vapor and the liquid fractions. The method shall be demonstrated with expanding wet steam. The result of the algorithm will be the discharge enthalpy, temperature and the discharge vapor fraction for given initial pressure and temperature, discharge pressure and polytropic efficiency. Based on the developed formulas some well-known but nevertheless paradoxically perceived results and comparisons with expanding dry air can be shown.

Study on Kelvin–Helmholtz Instability With Heat and Mass Transfer

The effect of heat and mass transfer on the Kelvin–Helmholtz instability between liquid and vapor phases of a fluid has been studied using three different theories: a purely irrotational theory based on the dissipation method, a hybrid irrotational-rotational theory, and an inviscid potential flow theory. These new results are compared with previous results from viscous irrotational theory. The stability criterion is given in terms of the critical value of relative velocity. The system is shown to be unstable when the relative velocity is greater than the critical value of relative velocity; otherwise, it is stable. It is observed that heat and mass transfer has a destabilizing effect on the stability of the system while vapor fraction has a stabilizing effect.

Thermal Optimization of a Solar Thermal Cooling Cogeneration Plant at Low Temperature Heat Recovery

A simple cooling cogeneration has been developed by coupling a Kalina cycle system (KCS) with a vapor absorption refrigeration (VAR) system. The working fluid used in this theoretical thermodynamic evaluation is ammonia water mixture. A low temperature heat recovery (150 °C–200 °C) from engine exhaust gas, solar collectors, or similar can be used to operate the plant. A controlling facility is provided to set the required amount of power or cooling to meet the variable demand. In this proposed plant, the liquid refrigerant absorbs more amount of heat from evaporator surroundings with a flow control located in between power and cooling cycles. The extra included components are condenser, heat exchanger and throttling device over KCS plant. Due to possibility of more cooling, it offers high energy utilization factor (EUF). The coupled plant characteristics are studied with changes in mass split ratio, separator vapor fraction, separator temperature, and turbine concentration to develop efficient working conditions. The power mass split ratio is varied from 80% to 100% to run the coupled plant at nearly full load conditions. The separator vapor fraction and temperature are optimized at 45% and 150 °C, respectively. It is recommended to maintain the turbine concentration above 0.85 for optimum power and cooling. The maximum cycle EUF and plant EUF are 0.15 and 0.06, respectively, at 80% power mass split ratio. The specific power and specific cooling at these conditions are 62 kW/kg and 72 kW/kg, respectively.

Numerical Simulation of Bus Windshield Based on Heat Transfer with Impinging Jets

The maximum vapor fraction in the air decreases with the temperature falling. For the air which has the high humidity, the redundant vapor will condense into fog or freeze into ice if the moist air attained a new saturation in a lower temperature. The buses running in the north cold areas usually face this problem in winter. According to the theory of jet impingement heat transfer, this thesis analyzed the influences of angle between the windshield and impinging jets and also the distance between vents and dashboard edge on defrosting by a simplified bus model. It also put forward some suggestions about defrosting on buses.

Dynamic Optimization of an Evaporator by a Nonlinear Model Predictive Controller for Operation at Modular Micro Rectification

Arranging micro-structured equipment to plants whole production processes can be realized with maximum efficiency in tightest space. Unit operations are thereby represented as individual functional modules in shape of micro devices. In a multi unit operation plant a correspondingly large number of manipulable variables have to be coordinated. Due to the design of micro-scaled devices plants form sophisticated systems, while for a fully optimized control still no common satisfying solutions exist. A system of modular, discontinuous phase contacting, micro rectification consists of unit operations heating, cooling, mixing and separating. Heat exchangers, mixers and cyclones for phase separation can be arranged to a counter-current rectification system with maximum mass-transfer efficiency every unit. Operating an electrical heated evaporator for modular rectification purposes a strong coupling of mass flow with the vapor fraction and the outlet temperature can be observed [4]. Operating at a predefined state for mass flow, temperature and vapor fraction may only be possible with difficulties using traditional methods of linear control technology. For dynamic optimization of the multivariable micro-structured evaporator principle of Nonlinear Model Predictive Control (NMPC) was generically formulated in C++ and implemented to LABVIEW 7. Every discrete time step an objective function is generated from nonlinear process models in the form of grouped NARX-polynomials. Optimal sequences of control actions for plant operation are evolved. The resulting constrained cost function is non-convex making detection of relative local optimum a difficult task. This obstacle can be gone around using heuristic optimization algorithm in combination with traditional techniques. Based on experimental results it was demonstrated that NMPC keeps the coupled variables mass flow and temperature energy saving with minimal control activity in the entire two-phase region on their set-points.