Three-dimensional Unsteady Heat Transfer Research Articles

A comparative analysis of the performance of backfill heat exchangers in deep mine geological environments

With the increase of mineral mining depth, the technologies of constructing heat exchanger in stopes to exploit geothermal resources from deep mines have received great attention. The heat extraction capacities of heat exchangers are closely related to the geological environments of deep mines, especially backfill mines. Therefore, a three-dimensional unsteady heat transfer and seepage coupling model of backfill heat exchangers (BFHEs) is created and verified in this work using COMSOL simulation software. Then, the influences of geological environments, involving groundwater seepage, initial temperature and thermal conductivity of surrounding rock etc., on the ability of five BFHEs with the equal buried tube length to extract heat in different configurations are investigated by this model. The findings indicate that groundwater flow can significantly improve the performance of BFHEs. Double-layer tube BFHEs have the best overall performance in the studied seepage velocity range. Only when there is no or little seepage is the performance of single-layer tube BFHEs equivalent to that of double-layer tube BFHEs. The heat extraction capacity of BFHEs is greatly improved under seepage conditions with the increase in the initial temperature of surrounding rock, although the impact is low in the absence of groundwater seepage. The benefit of backfill body's thermal conductivity on BFHEs' ability to transport heat is significantly greater than that of surrounding rock, but the effect becomes smaller as the value increases. Further increasing the thermal conductivity of the backfill body will not considerably increase the heat extraction capability of BFHEs after it reaches 2.5 (W·m−1·K−1). Therefore, if the cost is too high, continued enhancement is not recommended. The findings offer theoretical recommendations for optimizing BFHEs’ setup to better adapt to deep mine geological environments and increase geothermal extraction capability.

Thermal safety of solid rocket motor with complex charge structure

This present work is focused to study thermal safety and heat transfer characteristics of large-scale solid rocket motor (SRM) with complex star-charged structure. Here, we have used three-dimensional unsteady heat transfer model coupled with a two-step global reaction mechanism of ammonium perchlorate/hydroxyl terminated polyether (AP/HTPE) propellant. Comparisons of the ignition delay time and temperature of computational results are done with experimental studies, and a reasonable match has been obtained in these comparisons. It is found that the heats transferred from the thermal environment and released from the exothermic reaction of the propellant jointly intensify the occurrence of the cook-off response of the SRM. The propellant at the bottom of the SRM can reach to a higher temperature over the time and thus the exothermic reaction is rapidly intensified with an increase of the propellant temperature, releasing more reaction heat simultaneously. The heat energy is gradually accumulated in the SRM and the propellant is eventually ignited under this high-heat environment. The ignition temperature of the SRM basically linearly increases with the AP content at 10.8% HTPE and the ignition temperature curve under different AP contents is closer to the quadratic curve at 7.8% HTPE. Also, the content of HTPE mainly has a certain effect on the ignition time that is 288 s and 283 s with its content of 7.8% and 10.8%, respectively. The heat transfer rate and the case temperature of the SRM are all increasing with an increase of environmental temperature, while the depth of the heat transfer is decreasing. Further, the ignition delay time can be expressed on environmental temperature as: ti = 2.073263exp11 × Te-2.89.

Three-dimensional heat transfer characteristics of complex structure with nineteen tubular propellants under different heating rates

This work is focused to study heat transfer mechanism and cook-off response characteristics of the complex structure with nineteen tubular cluster double-base propellants. The three-dimensional unsteady heat transfer model with chemical reaction mechanism is developed to provide a detail means of capturing fundamental thermal phenomena of multiple materials occurring during heating process with different environmental heating rates. Comparisons of temperature variation of computational results are done with experiments, and a reasonable match has been obtained. Further, the cook-off response characteristics under slow heating conditions can be divided into two stage. The temperature rise in the first stage is dominated by the heat transfer mechanism between multiple martials with complicated structure, and the main factor in the other stage is chemical reaction effect of the double-base propellant that can release more reaction heat. The environmental heating rate has an obvious effect on the cook-off response parameters, including ignition temperature, delay time and response position. The results show that with an increase of heating rate, the cook-off response time exponentially decreases and can be expressed as: ti = 21.59exp(−logk/1.106)-0.07. Especially, the path of temperature rise has changed dramatically under the combined effects of different heat conductivities inside the motor and environmental heating rate.

Study on heat storage performance of a novel vertical shell and multi-finned tube tank

High-efficient latent heat storage technology plays a crucial role in solar energy utilization. In this study, a shell-and-tube heat storage tank employing a novel fin structure has been proposed. A three-dimensional unsteady heat transfer model of the tank has been established. The effects of fin height, fin angle and fin number on liquid fraction, average temperature and heat storage rate have been investigated. The results show that the complete melting time of the phase change material in the novel finned tube is shortened by 66.4% compared with the finless structure. The melting time of the fins with heights of 34.20 mm, 42.75 mm, and 51.30 mm are shortened by 56.9%, 60.1% and 66.4%, respectively. When the fin height exceeds 51.30 mm, the melting rate remains almost unchanged. The fin angle is not the larger the better. Compared with the bending angles of 10°, 20° and 40°, the complete melting time of 30° is reduced by 14.0%, 11.6% and 6.4%, respectively. As the number of fins increases, the total heat storage time is shorter, but the total heat storage has also been reduced. The results can provide a good reference for design, operating, and energy-saving of latent heat storage systems.

Numerical study on effect of natural wind and piston wind on anti-freezing length of tunnels with high geo-temperature in cold region

Taking Zilashan Tunnel as an example, a three-dimensional unsteady heat transfer model of surrounding rock, tunnel lining and wind flow was established. The reliability of the temperature calculation model was verified by a 1:30 model test and field test. The influences of natural wind speed, natural wind temperature, train speed and traffic volume on the anti-freezing length of tunnels in cold regions were studied. In particular, this paper focuses on the influence of piston wind direction on the anti-freezing length of the tunnel during 50 years of operation. The results show that the maximum anti-freezing length is positively correlated with the velocity of the natural wind, and the maximum anti-freezing length increases by 787 m on average when the velocity of the natural wind increases by 1 m/s. The maximum anti-freezing length is 2352 m when only the influence of natural wind is considered. When the average temperature of the natural wind is greater than 5 °C, the anti-freezing length of Zilashan Tunnel is 0 m. The maximum anti-freezing length increases with the increase of train speed. The anti-freezing length is the longest when the natural wind is in the same direction as the piston wind, and the minimum anti-freezing length is more than 1200 m under different train speeds. When the traffic volume is 20 trains/day, the anti-freezing length increases with the increase of train speed regardless of the direction of piston wind. Under most working conditions, the surrounding rock near the tunnel entrance will freeze; when the traffic volume is greater than 20 trains/day, the surrounding rock near the tunnel exit will freeze with the natural wind speed and train speed at 1.5 m/s and 200 km/h respectively.

Pyrolysis of single large biomass particle: Simulation and experiments

Pyrolysis and heat transfer characteristics of single large biomass particle were investigated using three-dimensional unsteady heat transfer model coupled with chemical reactions. The consumption of biomass and the production of products were simulated. Some experiments were designed to provide model parameters for simulation calculations. The simulation was verified by pyrolysis experiments of large biomass particle in a vertical tube furnace. The simulation results show the internal heat and mass transfer law during the pyrolysis of large biomass particle. When the biomass particle diameter is between 10 and 30 mm, for every 5 mm increase in particle diameter, the time required for complete pyrolysis will increase on average by about 50 s. When the pyrolysis temperature is between 673 K and 873 K, a slight decrease in the pyrolysis temperature will cause the time required for the biomass to fully pyrolyze to rise significantly. And the phenomenon is more obvious in the low temperature range. The results indicate that the numerical simulation agrees well with the experimental results.

Comparison of ground coupled heat transfer models for predicting underground building energy consumption

Underground buildings have gained attentions to alleviate urban land-scarcity. Evaluation of energy performance remains complex and uncertain for underground buildings, where the heat transfer through building envelopes is a typical three-dimensional unsteady heat transfer process. A few models have been proposed to calculate the ground coupled heat transfer (GCHT) through underground building envelopes. This paper evaluates and analyzes the influence of different GCHT models on energy consumption prediction of underground buildings. The models tested include: Ground Domain Xing (EnergyPlus), Basement Preprocessor (EnergyPlus), Foundation Kiva (EnergyPlus), and Frequency Response Analysis (DeST), which are compared under the criteria of consistency of model outputs and computational time. The study reveals that the Ground Domain Xing model can be considered as the relatively accurate GCHT model with low computational cost. One critical factor that differentiates these GCHT models is the ground surface boundary condition used. The study further explores the influence of thermal characteristics of underground building envelopes on annual energy consumption for five climate zones of China using the Ground Domain Xing model. The results show that the better thermal performance of underground building envelopes may increase the annual energy consumption in different climate regions.

Numerical simulation of moving heat source in arc welding using the Element-free Galerkin method with experimental validation and numerical study

A three-dimensional unsteady heat transfer model based on the meshless Element-Free Galerkin (EFG) method was developed to predict the temperature field generated by a moving heat source. Numerical results obtained from the EFG method were compared with those from the finite element method (FEM) and then validated with robotic arc welding experimental results. Good agreements were found between the numerical and experimental results. An EFG parametric study was also carried out to investigate the effects of (i) the size and shape of the support domain, (ii) the ratio of quadrature points to field nodes, and (iii) the different weight functions.

Numerical simulation of heat release performance of filling body under condition of heat extracted by fluid flowing in buried tube

It is the basic requirement of the synergetic exploitation of deep mineral resources and geothermal resources to arrange the heat transfer tube in filling body. The heat release performance of filling body directly impacts on the exploiting efficiency of geothermal energy. Based on heat transfer theory, a three-dimensional unsteady heat transfer model of filling body is established by using FLUENT simulation software. Taking the horizontal U-shaped buried pipe as research object, the variation of temperature field in filling body around buried pipe is analyzed during the heat release process of filling body; the initial temperature of filling body, the diameter of buried pipe, the inlet temperature and inlet velocity of heat transfer fluid influencing of coupling heat transfer, which exists between heat transfer fluid and surrounding filling body within a certain axial distance of buried tube, and influencing of temperature difference between inlet and outlet of heat transfer fluid and on heat transfer performance of filling body are also discussed. It not only lays a theoretical foundation for the synergetic exploitation of mineral resources and geothermal energy in deep mines, but also provides a reference basis for the arrangement of buried pipes in filling body as well as the selection of working conditions for heat transfer fluid.

Research of PWR Pressurizer Insurge Characteristics on Three-Dimensional Transient Modeling

In order to study the evolution process and hydraulic characteristics of pressurizer insurge in CPR1000 pressurized water reactor (PWR), the computational fluid dynamics (CFD) of three-dimensional unsteady heat transfer was used to capture the temperature and velocity fluctuation intensity of the mixing of the hot and cold. The results show that Realizable k–ε Turbulence Model combined with VOF multiphase heat transfer model can effectively predict the development trend of pressurizer insurge process. The small diameter pressurizer surge line of CPR1000 enhances the intensity of velocity fluctuation. From the essence of flow and heat transfer, it is concluded that buoyancy force can increase the degree of fluctuation and make an accelerated effect on the influx cold fluid. The electric heater inside the pressurizer should be arranged as far as possible in z<-0.45m and z>0.45m; it is beneficial to improve its harsh operating environment. This research can provide reference for the structural design of pressurizer and the layout optimization of the electric heater.

Three-Dimensional Simulation of Base Bleed Unit with AP/HTPB Propellant in Fast Cook-off Conditions

ABSTRACTIn this work, a three-dimensional unsteady heat transfer model of base bleed unit with trilobite ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB) composite solid propellant is presented to analyze the cook-off characteristics. According to the two-step chemical reaction of AP/HTPB propellant, a small-scale cook-off test is established. A comparison of the experimental and calculated results is made to verify the rationality of the computation model. On this basis, a cook-off numerical simulation of the base bleed unit at the heating rates of 0.33, 0.58 and 0.83 K/s is presented to investigate the ignition and initiation characteristics. The results show that the ignitions occur on the head face of the AP/HTPB propellant and near the internal gas chamber in these conditions. As the heating rate increases, the runaway time decreases and the ignition temperature rises.