Local Heat Transfer Processes Research Articles

Deep reinforcement learning-PID based supervisor control method for indirect-contact heat transfer processes in energy systems

Indirect-contact heat exchangers have been widely used in various energy systems, and the precise tracking control of important heat transfer parameters, such as temperature, is vital for safe and efficient operation. However, the high nonlinearity of heat transfer and large disturbance brings difficulty to optimal control. Considering the strong perception and decision-making capabilities of deep reinforcement learning (DRL), this study proposed a supervisor control method combined DRL and proportional–integral–derivative (PID). A set of the fewest conveniently measurable variables was derived as agent observations to describe the heat transfer process effectively and thereby improve the control efficiency under large disturbances. In addition, the local heat transfer process was used as a training environment to reduce training costs significantly. Finally, superheat temperature control in a complex organic Rankine cycle was simulated with SIMULINK to evaluate the effectiveness of the proposed observation variables and the training and control methods. The results showed that the proposed control method achieved satisfactory performance. The average absolute tracking error was only 0.246 K under trained and untrained disturbances, whereas that of the PID control was 4.645 K. Compared with the model predictive control, the DRL-PID-based supervisory control evidently performed better under a large disturbance; the average absolute tracking errors under DRL-PID control and MPC were 0.288 K and 0.509 K, respectively.

Coupling a Borehole Thermal Model and MT3DMS to Simulate Dynamic Ground Source Heat Pump Efficiency.

We present a novel integrated two-region model that couples simulation of local heat transfer processes in borehole heat exchangers (BHEs) with field-scale heat transport simulation using MT3DMS to fulfill the dynamic simulation of the borehole geothermal systems. This includes the prediction of subsurface thermal perturbation induced by BHEs, derivation of the U-pipe circulating fluid temperature profile within boreholes, and the evaluation of the ground source heat pump efficiency based on available time series of building heat load. In our approach, MT3DMS is the simulator for the two-dimensional field-scale heat transport, while new Python modules are developed to analytically solve the thermal transfer process within boreholes and interface iteratively with MT3DMS. A Python package for scripting MODFLOW-based code named Flopy is used to establish the MT3DMS numerical model. The proposed model is validated against analytical solutions and we demonstrate the application to more complex test problems with field-scale heterogeneity and pumping. Instructions are provided to access the source code and example problems which are available online.

A multi-scale CFD-system coupled code for transient analysis of the passive residual heat removal system of MHTGR

The passive residual heat removal system (PRHRS) is adopted by the HTR-10 and HTR-PM to remove the decay heat in the reactor core, so that the safety of the reactor can be guaranteed under accident conditions. A multi-scale CFD-system coupled code used to calculate the transient characteristics of the PRHRS of HTR-10 was developed in this study. With this coupled code, the transient characteristics of the PRHRS during the startup process were calculated. The results show that the computational results are in good agreement with the experimental results, which validates the accuracy of the coupled code. The ability of the coupled code to simultaneously calculate the local heat transfer process in the reactor cavity and the global heat removal characteristic of the PRHRS is also shown. Moreover, radiation heat transfer plays a very important role in the heat removal process of the PRHRS.

Local heat transfer under an array of micro jet impingement using HFE-7000

An experimental and numerical study was conducted to elucidate local heat transfer processes under multiple microscale jet impingements using a dielectric coolant, HFE-7000. The micro device used in this experiment was made of a 400-μm thick silicon wafer, a 210-μm thick vinyl sticker, and a 1-mm thick Pyrex substrate. Fourteen jet orifices were etched using deep reactive ion etching (DRIE) on the silicon wafer, and four 100-nm thick resistance temperature detectors (RTDs) and a heater were fabricated from titanium on the Pyrex substrate. The double-sided vinyl sticker was used to bond the layers and to form a micro fluidic channel having dimensions of 1.9 mm × 14.8 mm × 210 μm.Jet Reynolds numbers in the experimental study ranged from 162 to 4057 and nominal heat fluxes ranged from 10 W/cm2 to 80 W/cm2. A three-dimensional numerical model was developed to predict the jets hydrodynamics and the convection heat transfer coefficients using a turbulent flow model for the turbulent range of the flow. Good agreement was found between the numerical predictions and experiments. The numerical results also provided valuable insight into the flow patterns formed due to multiple jet interactions. Average Nusselt number and pressure drop coefficient values found in the present study correspond well with the existing correlations pertinent to jet array impingement.

Experimental Modelling of Local Heat Transfer Process for a Gas-liquid System in an Agitated Vessel with the System of A 315 – RT ImpellersExperimental Modelling of Local Heat Transfer Process for a Gas-liquid System in an Agitated Vessel with the System of A 315 – RT Impellers

Experimental Modelling of Local Heat Transfer Process for a Gas-liquid System in an Agitated Vessel with the System of A 315 – RT ImpellersExperimental Modelling of Local Heat Transfer Process for a Gas-liquid System in an Agitated Vessel with the System of A 315 – RT Impellers

Heat transfer scaling analysis of the single-phase natural circulation flow system

In this study, the heating power in the heat source or the core decay power is proposed as the reference parameter for the scaling analyses. From the scaling analyses assuming quasi-steady state flow, three system-level dimensionless numbers for the thermal heat transfer were obtained: the dimensionless power rate number, the dimensionless HX heat transfer number, and the dimensionless heat loss number. Based on the three local heat transfer processes, the dimensionless HX heat transfer number is further broken down into three dimensionless local heat transfer numbers: the dimensionless HX tube internal convective heat transfer number, the dimensionless HX tube conductive heat transfer number, and the dimensionless HX tube external convective heat transfer number, with each representing local heat transfer similarity. Analogically, three dimensionless local heat loss numbers were also derived. With the obtained dimensionless numbers, the scaling criteria for exact HX heat transfer similarity were derived. It was found the tube inner diameter should be scaled down in terms of the tube length scaling ratio for exact heat transfer similarity. In addition, three sets of scaling criteria for a full-pressure model, the new scaling criteria (general), the new scaling criteria (ideal), and the new scaling criteria (practical), were proposed.

Conjugate heat transfer modeling and asymmetric characteristic analysis of the heat collecting element for a parabolic trough collector

We analyzed in this study the conjugate heat transfer process in the heat collecting element (HCE) for a parabolic trough concentrator. The effect of asymmetric characteristics, such as non-uniform heat flux, eccentric configuration, and incident angle, are especially considered in this paper. We constructed a two-dimensional model of surface radiative transfer coupled with convection using the finite volume method combined with the net radiation method. We presented a detailed numerical method in which we applied an additional source term method to the boundary conditions to capture the absorption and emission processes in the coating of the HCE. The results were validated against the experimental data. Thus, we were able to identify the limiting interactions between thermal radiation and convection in the annulus. We systematically analyzed the effect of the incident irradiation angle and the eccentricity of the configuration on heat loss through the annulus of the HCE under two limiting conditions: “air in annulus” and “vacuum in annulus”. The results show that asymmetric characteristics, especially the non-uniform heat flux and eccentricity configuration, have a significant influence on local heat transfer processes. These insights may be used to further optimize the configuration of the HCE for improved thermal performance.

Local Heat Transfer Process for a Gas–Liquid System in a Wall Region of an Agitated Vessel Equipped with the System of CD6-RT Impellers

The results of the experimental studies of local heat process for a gas–liquid system in the region of the cylindrical wall of an agitated vessel equipped with the system of CD 6–RT impellers are presented. A lower CD 6 impeller (Smith turbine) and upper RT impeller (Rushton turbine) were located on the common shaft in a baffled agitated vessel of inner diameter equal to 0.3 m. Liquid height in the agitated vessel was equal to 0.6 m. Newtonian liquids of different physical properties were used as a continuous phase. Air was dispersed in the liquid. Local heat transfer coefficients were measured using both thermal and electrochemical methods. In total, 2280 experimental points were obtained. Distributions of the heat transfer coefficients were described by means of eqs 12–17 and 18–23 as a function of the Re and Pr numbers, dimensionless axial coordinate z/H, and modified Frg number, separately, for turbulent and transitional ranges of the fluid flow in the agitated vessel. Equations 12–17 and 18–23, concerning both coalescing and noncoalescing gas–liquid systems, have no equivalent in the open literature.

Scaling analysis of coolant spraying process in automatic depressurized system

Through the automatic depressurized system of AP1000 reactor, the steam with high temperature and high pressure from the pressurizer sprays into the water tank, and the pressure in the primary coolant system will diminish to protect the reactor. However, as the steam continues to flow into the tank, the water temperature rises rapidly until boiling occurs, which will affect the local heat transfer process. Therefore, it is necessary to understand the corresponding heat transfer mechanism by means of experiment. To ensure that the experimental results with a scale-down model reflect the actual situations of the prototype we analyzed the process of steam jetting from the pipeline into the water tank, and summarized scaling rules based on equation analysis method, including various flow stages. The results show that the phenomena-based similarity between the model and the prototype should meet: (1) geometrically similar model and prototype; (2) equal thermal parameters and identical initial conditions, which can greatly simplify other similarity parameters; (3) at the blowdown stage, keep the steam mass flux and the nozzle diameter consistent; (4) at the natural convection stage with single phase fluid, the equality in terms of Prandtl number and the Grashof number should be met first, while assessing the relative uncertainty.

Viscous elongation of glass rods: Experiments, simulations, and analysis

Viscous elongation of glass rods has been investigated to improve procedures for producing parabolic capillaries from straight capillaries for use as intensity concentrating x-ray optical elements. Experiments and simulations have been carried out pulling Corning 7740 borosilicate glass rods in a cylindrical furnace. Simulations were based on one-dimensional and two-dimensional finite difference/element calculations, assuming that deformation occurred by linear viscous flow and that the temperatures in the glass remained in equilibrium with the temperatures measured along the axis of the furnace. Rates of elongation in experiments were smaller than those in simulations for later stages of the elongation process. Further experimental and analytical investigations ruled out explanations based on increased viscosities caused by structural relaxation or by partial crystallization and showed that the simulations gave incorrect results because they did not incorporate the dependence of local heat transfer processes on the glass rod radius.

Local heat transfer process and pressure drop in a micro-channel integrated with arrays of temperature and pressure sensors

One of the most important components in micro-fluidic system is the micro-channel which involves complicated flow and transport process. This study presents micro-scale thermal fluid transport process inside a micro-channel with a height of 37 μm. The channel can be heated on the bottom wall and is integrated with arrays of pressure and temperature sensors which can be used to measure and determine the local heat transfer and pressure drop. A more simplified model with modification of Young’s Modulus from the experimental test is used to design and fabricate the arrays of pressure sensors. Both the pressure sensors and the channel wall use polymer materials which greatly simplify the fabrication process. In addition, the polymer materials have a very low thermal conductivity which significantly reduces the heat loss from the channel to the ambient that the local heat transfer can be accurately measured. The air flow in the micro-channel can readily become compressible even at a very low Reynolds number condition. Therefore, simultaneous measurement of both the local pressure drop and the temperature on the heated wall is required to determine the local heat transfer. Comparison of the local heat transfer for a compressible air flow in micro-channel is made with the theoretical prediction based on incompressible air flow in large-scale channel. The comparison has clarified many of the conflicting results among different works.

Bulk heat transfer coefficient in the ice–upper ocean system in the ice melt season derived from concentration–temperature relationship

The bulk heat transfer coefficient in the ice–upper ocean system (Kb) in the ice melt season is estimated by a new method at 18 areas that cover much of the Antarctic seasonal ice zone. The method is based on a model in which ice melting is caused only by heat input through open water and is treated in a bulk fashion in the ice–upper ocean system. Kb is estimated by fitting a convergent curve derived from the model to an observed ice concentration–temperature plot (CT‐plot). Estimated Kb is 1.15 ± 0.72 × 10−4 m s−1 on average. If Kb can be expressed by the product of the heat transfer coefficient (ch) and the friction velocity (uτ), ch is 0.0113 ± 0.0055. This value is about two times larger than that estimated at the ice bottom. The relationship between Kb and the geostrophic wind speed (Uw), which is roughly proportional to uτ, shows a significant positive correlation, as expected. Further, Kb seems more likely to be proportional to the square or cube of Uw rather than a linear relationship. Since Kb estimated from our method is associated with ice melting in a bulk fashion in the ice–upper ocean system, this relationship likely indicates both the mixing process of heat in the upper ocean (proportional to uτ3) and the local heat transfer process at the ice–ocean interface (proportional to uτ).

Thermal chip fabrication with arrays of sensors and heaters for micro-scale impingement cooling heat transfer analysis and measurements

The design and fabrication for a thermal chip with an array of temperature sensors and heaters for study of micro-jet impingement cooling heat transfer process are presented. This thermal chip can minimize the heat loss from the system to the ambient and provide a uniform heat flux along the wall, thus local heat transfer processes along the wall can be measured and obtained. The fabrication procedure presented can reach a chip yield of 100%, and every one of the sensors and heaters on the chip is in good condition. In addition, micro-jet impingement cooling experiments are performed to obtain the micro-scale local heat transfer Nusselt number along the wall. Flow visualization for the micro-impinging jet is also made. The experimental results indicate that both the micro-scale impinging jet flow structure and the heat transfer process along the wall is significantly different from the case of large-scale jet impingement cooling process.

Heat exchanger fabrication with arrays of sensors and heaters with its micro-scale impingement cooling process analysis and measurements

The design and fabrication for the thermal chip of a heat exchanger with arrays of temperature sensors and heaters for study of micro-jet impingement cooling heat transfer process is presented. This thermal chip can minimize the heat loss from the system to the ambient and provide a uniform heat flux along the wall, thus, local heat transfer processes along the wall can be measured and obtained. The fabrication procedure presented can reach a chip yield of 100%. With fabrication of a micro-slot nozzle of B=200 μm, therefore, micro-jet impingement cooling experiments can be performed to obtain the local micro-scale heat transfer Nusselt numbers along the wall. The experimental results indicate and the heat transfer process along the wall is significantly different from the case of large-scale jet impingement cooling process. Correlations for the micro-scale heat transfer cooling process have been successfully obtained.