•A Robust Model Predictive Control for ICE thermal management was developed.•The proposed control is effective in decreasing the warm-up time.•The control system reduces coolant flow rate under fully ...warmed conditions.•The control strategy operates the cooling system around onset of nucleate boiling.•Little on-line computational effort is required.
Optimal thermal management of modern internal combustion engines (ICE) is one of the key factors for reducing fuel consumption and CO2 emissions. These are measured by using standardized driving cycles, like the New European Driving Cycle (NEDC), during which the engine does not reach thermal steady state; engine efficiency and emissions are therefore penalized. Several techniques for improving ICE thermal efficiency were proposed, which range from the use of empirical look-up tables to pulsed pump operation. A systematic approach to the problem is however still missing and this paper aims to bridge this gap.
The paper proposes a Robust Model Predictive Control of the coolant flow rate, which makes use of a zero-dimensional model of the cooling system of an ICE. The control methodology incorporates explicitly the model uncertainties and achieves the synthesis of a state-feedback control law that minimizes the “worst case” objective function while taking into account the system constraints, as proposed by Kothare et al. (1996). The proposed control strategy is to adjust the coolant flow rate by means of an electric pump, in order to bring the cooling system to operate around the onset of nucleate boiling: across it during warm-up and above it (nucleate or saturated boiling) under fully warmed conditions. The computationally heavy optimization is carried out off-line, while during the operation of the engine the control parameters are simply picked-up on-line from look-up tables. Owing to the little computational effort required, the resulting control strategy is suitable for implementation in the ECU of a modern engine.
The control strategy was validated by means of experimental tests under several operating conditions, involving both warm-up and fully warmed engine thermal states. The tests were carried out with a small displacement Spark-Ignition Engine which was equipped with an electric coolant pump, directly driven by the control algorithm.
Results show that the controller is robust in terms of disturbance rejections, it respects the defined system constraints and is also very fast in terms of response to the perturbations. The experimental tests proved that the proposed control is effective in decreasing the warm-up time and in reducing the coolant flow rate under fully warmed conditions as compared to a standard configuration with pump speed proportional to engine speed. The adoption of these cooling control strategies will, therefore, result in lower fuel consumption and reduced CO2 emissions.
•A new DNB-type CHF prediction model of flow boiling in rifled tube at low qualities was established.•A new αb correlation was proposed based on a large number of CHF experimental data.•The present ...model was verified by the CHF look-up values and CHF experimental values.•Three classic CHF prediction models and the present CHF model were evaluated.•With the increase of vapor quality, the influence of pressure and mass flux on CHF weakened.
Based on the near-wall bubble coalescence model and the structural characteristics of the rifled tube, improvements and optimization of the model for rifled tube were conducted. DNB-type CHF phenomenon in the rifled tube under subcritical and near-critical pressures was investigated by numerical calculation. A new CHF prediction model of flow boiling in rifled tube with low qualities was established and compiled by FORTRAN language. The model used the Staub model which considering the influence of the bubble contact angle to calculate the bubble detachment diameter. The surface friction coefficient of the rifled tube was calculated by the classical Kohler and Kastner formula. And a new critical void fraction of the wall bubbly layer αb correlation was proposed based on a large number of CHF experimental data. The present model was verified by the CHF look-up values and CHF experimental values. Based on the CHF experimental data of the rifled tube, three classic CHF prediction models and the present CHF model were evaluated, and it was found that the present modified CHF model has the highest prediction accuracy for the rifled tube, which is the best agreement with the experimental values, indicating that the present model is appropriate for DNB-type CHF prediction of the rifled tube. In this paper, the effects of pressure, mass flux and inlet fluid subcooled enthalpy on CHF were studied. It was found that, the effect of pressure and mass flux on CHF is obvious in the low quality region, with the increase of vapor quality, the influence of pressure and mass flux on CHF weakens.
Critical Heat Flux (CHF) prediction remains one of the most important tasks in thermal hydraulics of Light Water Reactors (LWR’s). During recent years, tremendous progress has been made in ...understanding and predicting rod bundle CHF. However, due to the unique features associated with rod bundle subchannel systems, including complex and compact anisotropic geometry, non-uniform axial/radial heating, open channel interactions, periodic (positional) mixing vane grids (MVG’s) mixing (Yang et al., 2014), and a broad range of operation conditions (Yang and Han et al., 2019), a precise prediction of CHF in rod bundles, entirely based on analytical methods without the support of any extensive experimental database, is still beyond the present modelling and computational capabilities. For Pressurized Water Reactors (PWR’s), CHF occurs at a relatively low coolant quality and is primarily driven by the Departure from Nucleate Boiling (DNB) type event, which can be triggered by liquid sublayer dryout, bubble crowding, homogeneous nucleation, or other mechanisms causing local starvation of liquid under subcooled flow boiling or low quality saturated boiling conditions, and the dryout event, which describes CHF under a high void situation when liquid film on the fuel surface dries out, usually happens in a high quality saturated annular flow. Boiling Water Reactors (BWR’s) operate at a relatively lower pressure and can reach much higher coolant qualities that correspond to annular two-phase flow conditions. This dominant CHF mechanism, dryout, is based on the gradual thinning and drying out of the liquid film.
This paper provides an overview of the progress made in the experimental and analytical investigations of CHF in both PWR and BWR rod bundles during the past 40 years. A complete comprehensive concept map is developed to outline most of the key mechanistic CHF models covering the entire range of flow regimes. The limitation and the potential approaches for the application of mechanistic models to rod bundle CHF prediction are reviewed. This includes the recently proposed, rod bundle specific NHNM (Non-uniform heater Homogeneous Nucleation Model Yang, 2017). Various industrial empirical correlations are reviewed together with their corresponding subchannel codes and required rod bundle CHF experiments. The need for clear guidelines and standards for benchmarking new rod bundle CHF test facilities is discussed.
Various approaches on investigating, understanding, modeling, and predicting the occurrence of rod bundle CHF are reviewed from mechanistic models, semi-empirical and empirical approaches to advanced CFD simulation. The achievements and limitations in those methods are discussed with suggestions for future advancement.
•We propose a new heat transfer model and boiling model for the porous medium in PWRs.•We validate the model against experimental data under the simulated PWR conditions.•The liquid convection in the ...deposits contributes significantly to heat transfer.•The effect of permeability on heat transfer is found to be insignificant.
The model was developed to predict the heat transfer within the Chalk River Unidentified Deposit (CRUD), which is a kind of fouling found on the fuel rods of pressurized water reactors (PWR). The CRUD tends to develop steam chimneys that separate the liquid from the vapor phase. Therefore, the model describes the CRUD as a porous medium with steam chimneys. Unlike the previous approaches that assume that the evaporation takes place at the lateral surfaces of the chimneys, in the present study it is postulated that the vapor is generated by the bubble nucleation at the CRUD–clad interface, as observed via the visualization study for the chimney-structured porous medium. The generated bubble escapes through the steam chimney. The heat transfer in the CRUD can be described by three mechanisms of heat removal, which are nucleate boiling, liquid convection in the CRUD, and forced convective heat removal from the surface of the CRUD. The predicted CRUD–clad interface temperatures and overall heat transfer coefficients were compared to the experimental results, which were produced under the simulated PWR conditions (approximately 15MPa, 300°C). The prediction data presented better agreement with the experimental data; the normalized Root Mean Square Error (RMSE) of the present model is 18.6% in contrast with the 42.4% obtained with the Cohen model. After the validation with the experimental data, the prediction of temperature in the model was used to investigate how the heat transfer characteristics tended to change within the CRUD. Furthermore, the parametric study regarding the CRUD properties revealed that the effect of permeability on heat transfer is not significant in the nucleate boiling regime.
For several decades, a porous surface has been recognized as an efficient medium to increase boiling performance in a nucleate boiling regime. Most feasible porous surfaces have been studied in ...millimeter and micron-sized domains. It has been believed that a higher wall superheat is required to commence incipient nucleate boiling under a submicron regime. In this study, we demonstrate that a significantly enhanced pool boiling heat transfer is observed in a submicron regime through three dimensionally interconnected hybrid pores: the Alumina sponge-like nano-porous structure (ASNPS). The structural uniqueness of the ASNPS leads to an enlarged surface area, increases the potential number of the active nucleation site density, and improves the vapor–liquid menisci through the reentrant pore. Simultaneously, by changing the surface wettability with a hydrophobic self-assembled monolayer (SAM) coating, the number of active nucleation site density is improved. Eventually, the combination of the ASNPS and hydrophobic SAM coating can achieve substantial heat transfer coefficient (HTC) enhancement in the nucleate boiling. Also, the thickness of the ASNPS is a critical issue to adequately augment the HTC in pool boiling. The thickness of the ASNPS is optimized by examining the boiling performance of the ASNPS fabricated in different amounts of anodizing times. A classical mechanistic model from literature was modified and compared with the experimentally obtained data. The modified mechanistic model – with the combination of forced-convection and thin liquid film evaporation – showed reasonable predictions.
•Developing of a modified version of the RELAP5/Mod3.3 code.•Application of improved tool for heat transfer in passive in-pool safety systems.•Focus on evaluation of HTC in nucleate pool boiling and ...in film condensation.•Single effect validation process highlights improved capabilities of the code.•Integral analysis assesses significant improvements to simulate isolation condenser.
Despite system thermal–hydraulic codes were extensively validated for transient simulations of LWR, several activities highlighted limited capabilities of these tools to model heat transfer within in-pool passive power removal system. Discrepancies with experimental results were related to the underestimate of pool boiling and film condensation heat transfer coefficients. Thus, the DIAEE of “Sapienza” University of Rome developed a modified version of RELAP5/Mod3.3, able to handle fundamental heat exchange phenomena involved in passive in-pool safety systems. A primary validation procedure has been performed for separated and integral effects. Dealing with nucleate boiling, the Root Mean Squared Relative Error (RMSRE) of wall superheat has been reduced from 1.290 to 0.182. Concerning film condensation, wall temperature RMSRE has been reduced from 0.192 to 0.058. The integral effect assessment has involved an experimental test of the PERSEO facility. The qualitative comparison between experiments and calculations has highlighted significant improvements of the modified RELAP5/Mod3.3.
Abstract Accurate modeling of cryogenic boiling heat transfer is vital for the development of extended-duration space missions. Such missions may require the transfer of cryogenic propellants from ...in-space storage depots or the cooling of nuclear reactors. Purdue University in collaboration with NASA has assembled a database of cryogenic flow boiling data points from heated-tube experiments dating back to 1959, which has been used to develop new flow boiling correlations specifically for cryogens. Computational models of several of these experiments have been constructed in the Generalized Fluid System Simulation Program (GFSSP), a network flow code developed at NASA’s Marshall Space Flight Center. The new Purdue-developed correlations cover the full boiling curve: onset of nucleate boiling, nucleate boiling, critical heat flux, and post-critical heat flux boiling. These correlations have been coded into GFSSP user subroutines. The fluids modeled are nitrogen and methane. Predictions of wall temperature are presented and compared to the test data.
•Evaporation heat transfer of refrigerants in a plate-fin heat exchanger was investigated.•Adiabatic and evaporation flows of vertical upward flow were visualized.•Effects of mass flux, quality, and ...heat flux on evaporation flow were validated.•Heat transfer coefficients of R1234ze(E) and R32 were compared.•Heat transfer characteristics at local positions in the channel were investigated.
This study experimentally investigated the evaporation heat transfer and two-phase flow characteristics of a vertical upward flow using R1234ze(E) and R32 as test refrigerants in a plate-fin heat exchanger with offset fins. The adiabatic and evaporation flows of R1234ze(E) were visualized using a high-speed camera. Owing to the asymmetry of the inlet and outlet of the channel, a non-uniform velocity profile in the channel-width direction was observed under both adiabatic and evaporation flows. Evaporation flow accompanied by nucleate bubble generation at the lower area of the channel and the occurrence of a dry area at the upper side of the channel were observed. The heat transfer coefficients of R1234ze(E) and R32 were measured, and the effects of mass flux, quality, and heat flux on heat transfer were clarified. The heat transfer was strongly affected by forced convection at higher mass fluxes and quality whereas nucleate boiling dominated at higher heat fluxes and lower quality. The difference in the heat transfer characteristics at local positions in the channel was also investigated by measuring the local wall temperatures. Comparing refrigerants, the heat transfer coefficient of R32 was higher than that of R1234ze(E), regardless of the mass flux and quality, especially at lower mass flux conditions.
The basic objective of the paper is to identify the desired circulation ratio for the natural circulation of water tube boilers in different operating conditions. This requires the basic study of ...heat flux and the mode of the boiling heat transfer, and the phenomenon like departure from nucleate boiling and tube overheating. The parameters, which need to be studied are heat flux, pressure, dryness fraction, void fraction, liquid velocity and their impact on the required circulation ratio. For a natural circulation boiler, the circulation ratio is one of the most important design parameters as the other design parameter like critical heat flux and skin temperature are mainly derived from the circulation ratio. The required circulation ratio can vary with the boiler pressure, liquid velocity and maximum heat flux. This study is intended to provide input for the safe and optimum design of a natural circulation boiler.
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•Metal-organic deposition (MOD) nickel ink surfaces.•Nickel surface patterning performed under the magnetic field.•Increment by 50% in critical heat flux and 105% in heat transfer ...coefficient.•Roughness and wettability enhancement due to patterned Ni Structure.
We present high boiling heat transfer properties achieved by magnetically aligned nickel precursor inks on copper substrates measured atmospheric pressure. Alignment was performed in a single direction and grid orientation. Pool boiling studies were performed to obtain correlations between the heat flux, heat transfer coefficients, and wall superheat using water and ethanol. The effects of surface wettability and roughness on nucleate boiling heat transfer properties and bubble dynamics are reported. Our studies yielded a critical heat flux (CHF) of 185.9 W/cm2 and a heat transfer coefficient (HTC) of 106.9 kW/m2 °K for water on Ni grid patterned surface, representing an improvement of 49.9% in CHF and 105% in HTC compared to plain copper surface. The alignment of nickel followed by its sintering introduced nucleation characteristics that improved bubble dynamics. This is attributed to a) altered three-phase contact angle via chemically heterogeneity of Ni/Cu surface, and b) increased surface roughness due to nickel microstructures that induced wickability.
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