Low Ambient Temperature Conditions Research Articles

Heating performance of a vapor compression heat pump cascaded with a thermoelectric heat pump

Air source heat pumps (ASHPs) are widely utilized for heating and cooling in residential buildings; however, their effectiveness in heating mode is compromised during extreme weather conditions. Extensive research endeavors have been undertaken to develop, test, and assess a cost-effective vapor compression ASHP suitable for cold climate regions. This study takes an innovative approach by developing component and system prototypes for a cold climate heat pump. This design combines a thermoelectric heat pump (TEHP) with a traditional residential split ASHP to augment heating capacity in low ambient temperature conditions. The component and system prototypes underwent experimental testing in the psychrometric chambers. The experimental findings revealed a 13.6 % to 13.7 % increase in total heat pump heating capacity, accompanied by a 3.1 % to 5.0 % decrease in the coefficient of performance (COP) at ambient temperatures of −15 °C and −19 °C, when compared to the original ASHP. The COP of the TEHP is relatively constant, ranging from 1.63 to 1.76. This prototype offers a solution to address the challenges associated with reduced heat pump capacity at low ambient temperatures. The experimental results indicated that the lower the ambient temperature, thermoelectric heat pump auxiliary heating can increase the heating efficiency 60 % in comparison to electric resistance.

Numerical study of wall cooling effects on diesel spray combustion characteristics under different ambient conditions

The effects of wall cooling on diesel spray combustion characteristics and engine performance are varied under different environmental conditions and the reasons remain to be further analyzed. The systematic explanation of its influence laws can effectively help the cooling system design and fault isolation under changing working conditions. Therefore, the effect mechanism of wall temperature on the diesel spray combustion characteristics under different ambient temperature and pressure conditions is numerically revealed through the combination of CFD simulation and chemical reaction kinetics analysis. During the calculation model validation, the simulation results showed good agreement with the experimental results and good adaptability to the change in wall temperature. The results show that high ambient temperature and pressure lead to the acceleration of the reaction rate, and high pressure also slows down the fuel diffusion, which eventually causes the fuel to undergo more reaction processes before hitting the wall and being in the relatively late stage of the low-temperature reaction process. In the process of low-temperature reaction and heat accumulation, the effect of temperature drop in the late stage of the reaction is less than that in the early stage due to more heat accumulation before cooling. This ultimately leads to diesel ignition being delayed by low wall temperatures under low ambient temperature and pressure conditions, while diesel spray at high ambient temperatures and pressures has a stronger anti-interference ability to the wall temperature.

Features of a High-Stiffness Tire Interaction with a Bearing Surface During the Starting Period of Motion

The article emphasizes the importance and necessity of studying the behavior of automobile tires during operation in the starting mode, from the beginning of driving on “cold” tires to stabilizing its temperature and internal pressure. During this period, the main performance characteristics of the tire can change in a relatively wide range. Therefore, the main focus was on the initial period of driving as the most dangerous from the point of view of predicting the behavior of automobile tires. This article presents the results of analyzing a car tire’s condition and behavior during the starting movement. Features of the main parameter for assessing the stiffness characteristics of the tires were investigated. The research was conducted under conditions of low ambient temperatures during the operation of automobile tires. A numerical-analytical approach was used to estimate the stiffness parameters. Simultaneously, the initial data required for a correct analysis were obtained from the experimental results in actual road conditions. The obtained results allow for providing recommendations on the peculiarities of the automobile tires’ operation under adverse conditions, such as low ambient temperatures.

Real-time multi-variable optimization of the interstage subcooling vapor-injection based transcritical CO2 HPWH: An integrated model predictive control approach

The interstage subcooling vapor-injection technique has been introduced to transcritical CO2 heat pumps to alleviate performance degradation under conditions of low ambient temperature and high water-outlet temperature. This type of solution holds considerable appeal and finds utility across a wide range of applications. However, the efficacy and efficiency of such systems are contingent upon their management, which frequently requires the implementation of suitable control systems. The primary objective is to enhance the overall system operational performance, resulting in tangible advantages in terms of economic viability, energy conservation, and environmental sustainability. To achieve these objectives, one can utilize both explicit and implicit optimization methods ranging from Model Predictive Control (MPC) to Extremum Seeking Control (ESC), each with its strengths and weaknesses. This paper compares the performance of an ad hoc integrated MPC strategy and a multi-variable ESC one. Based on the results of the system operation employing the latter strategy, a significant linear correlation was discovered between the optimal medium pressure and the optimal discharge and suction pressure. Consequently, an intrinsic self-optimization strategy for the medium pressure was developed. Subsequently, a dynamic system identification was performed on the new system, incorporating this proposed strategy. To optimize the system Coefficient of Performance (COP) while maintaining the water outlet temperature, ensuring the thermal load constraint, the MPC strategy was employed. The main focus of the MPC was to determine the optimal Electronic Expansion Valve (EEV) opening degree and water pump settings. The integrated MPC strategy was evaluated and compared to the multi-variable ESC strategy under various conditions, including fixed design conditions, changing ambient conditions, and step-change water outlet temperature. The results clearly demonstrated the effectiveness and superiority of the integrated MPC.

Start-up characteristics of a R290 rotary compressor for an air source heat pump under low ambient temperature condition

Due to its excellent thermodynamic properties, low GWP refrigerant R290 is found to be promising alternative refrigerant. However, due to its special physical properties, high oil discharge rate and throttle valve blockage may occur for the R290 heat pump system under low temperature start-up conditions. In this paper, the transient characteristics, including the temperature, heat exchange rate, phase change rate, oil discharge rate and the liquid level of the oil sump, of a R290 rotary compressor used in the heat pump system under low temperature conditions are theoretically and experimentally studied. A transient heat and mass transfer model in the compressor is proposed, which considers the phase change of refrigerant in the shell. The experiment was carried out on the start-up characteristics of the compressor and the mathematical model was verified. The results show that high rotation speed and low temperature are the main reasons for the condensation of R290 in the shell. The excessive liquid refrigerant in the shell leads to a high oil discharge rate of up to 22 %. This paper will provide guidance for the use of R290 refrigerant under low temperature conditions.

Driving range evaluation of electric vehicle with transcritical CO2 thermal management system under different battery temperature controls

Improper battery temperature will lead to reduced battery discharge efficiency and electric vehicle driving range. Endeavors to find an efficient and precise battery temperature control method for the transcritical CO2 thermal management system of electric vehicles, two evaporation temperature control methods for battery cooling were proposed. First, the effects of the control methods on battery temperature, optimal discharge pressure and coefficient of performance (COP) were simulated under typical operating conditions. Subsequently, a comparative simulation was conducted on the battery temperatures and COP of the two control methods under full cooling conditions. The results revealed that equal evaporation temperature control induced a reduction in battery temperature to 11 °C (suboptimal levels) under conditions of low ambient temperature and vehicle speed. Conversely, while unequal evaporation temperature control could maintain the battery temperature within the optimal range of 20–45 °C, it would result in a maximum COP drop of more than 20% under certain conditions. Furthermore, the effects of the two control methods on driving range under different ambient temperatures and vehicle speeds were studied. The reasults indicated that the ultimate influence of uneuqal evaporation temperature control on the quarterly average driving range was virtually negligible, peaking at 1.35%. Given the dual-objective constraints pertaining to the driving range and the battery cooling performance, it was concluded that uneuqal evaporation temperature control was the optimal choice for direct battery cooling.

Optical Study on Spray and Two-Stage Ignition Characteristics for Diesel Spray Under Low Ambient Temperature and Density Conditions

Optical Study on Spray and Two-Stage Ignition Characteristics for Diesel Spray Under Low Ambient Temperature and Density Conditions

Experimental investigation of a post-combustion pool fire phenomenon due to the spray-wall interaction under DI engine-like cold-start conditions

Spray-wall interactions (SWI) have remained nowadays as an active area of research and the complex multi-physical field processes between the spray and the wall are still not well understood. In this study, a post-combustion pool fire phenomenon due to the SWI was found by optical diagnostic techniques: pool fire appeared near the stagnation point after spray combustion. The spray combustion and pool fire are not continuous. This abnormal combustion characteristic under low ambient temperature conditions (680–720 K) was investigated and the effect of ambient temperature on the macroscopic characteristics of pool fires was quantified and analyzed. The results show that the wall-attached fuel film will be re-ignited and continue to combust for a longer period after the impingement spray combustion. The appearance moment, dwell time, fire duration, and macroscopic characteristics of this post-combustion pool fire are non-monotonic as a function of the ambient temperature. When the ambient temperature is 700 K, the post-combustion pool fire appears earliest, the dwell time is the smallest, and the pool fire duration, flame length, and flame height of the pool fire are the largest. This is mainly dominated by the gas temperature around the wall-attached fuel film and fuel film mass.

Optimization design and performance analysis of ammonia heat pipe air heater under low ambient temperature conditions

In coal-fired power plants, steam-based air heaters consume high-quality steam and are easily damaged due to water freezing under low ambient temperature conditions. In this paper, we propose an ammonia heat pipe air heater design with hot water in power plants as the heat source, which can eliminate the risk of freezing and save high-quality steam. Based on distributed parameter model and hybrid genetic particle swarm optimization algorithm, an ammonia heat pipe air heater with one-pipe diameter is designed and optimized. The results show that the optimal one-pipe diameter design consumes much less power and owns less weight than the normal empirical design. A new and optimal two-pipe diameter design (i.e., multi-scale design) is further proposed, which drops the power consumption by 10.1% and the weight by 9.4% compared to the optimal one-pipe diameter design. The two optimal designs require different geometrical parameters, especially the pipe diameter(s), the pipe length, the number of heat pipes, and the air side longitudinal pitch. In addition, the two optimal designs provide slightly higher outlet air temperature uniformity than the non-optimal design. Of the three working fluids, i.e., ammonia, methanol, and ethanol, the ammonia heat pipe air heater performs best.

On the question of secondary use of exhaust exergy in power units of vehicles and tractors

BACKGROUND: Efficiency of modern internal combustion engines is no more than 40%, while the significant part of exergy, released during fuel combustion, dissipates in the environment through exhaust, coolant liquid and heated details. Meanwhile, the urgent problem of operating a tractor in low ambient temperature conditions is sustaining of optimal thermal condition for the sake of decreasing of losses, increasing of combustion efficiency and, as a consequence, increasing of operating efficiency.
 AIMS: Efficiency assessment of use of the complex system of utilization of thermomechanical exergy of diesel engine exhaust. Justification of type and number of stages of exhaust exergy utilization for diesel engines of vehicles and tractors for a wide range of ambient temperatures and loading conditions.
 METHODS: To accomplish given aims, theoretical and experimental studies were carried out in order to assess exhaust exergy potential in dependence on engine loading conditions and ambient temperature. Comparative studies of the D-440 and the SMD-62 engines were carried out, making it possible to assess the influence of exhaust system design features on amount of exergy losses.
 RESULTS: Exhaust performance losses of the SMD-62 engine with the TKR-11N-1 turbocharger at nominal mode were 0.8. Meanwhile, maximal exhaust performance losses at idle mode are 0.92. Authors suggested the special method of analysis of certain exhaust exergy components, which allows analyzing possible ways of its secondary use. The obtained equations determine quantitative and qualitative ratios between exhaust exergy components in comparison to maximal exhaust exergy potential to be utilized.
 CONCLUSIONS: Based on experimental data analysis and its statistical processing, it follows that middle powered engines have limited resources of waste heat. Theoretically, it should be enough for ensuring the turbocharger operation and sustaining of thermal condition in a gearbox and a cooling system in winter.

Experimental study of the effect of physical and chemical properties of alcohols on the spray combustion characteristics of alcohol-diesel blended fuels

To reveal the effect of different long carbon-chain alcohols blended into diesel on combustion characteristics and soot emissions under different ambient temperatures, the spray combustion processes of diesel and four sets of blends (the volume ratio of propanol/diesel, butanol/diesel, pentanol/diesel, and hexanol/diesel being 20%/80%) were investigated in a constant volume combustion chamber (CVCC) using optical techniques. It was discovered that the physical (cooling effect, dilution effect, diesel evaporation inhibition) and chemical reaction properties of alcohols have a competitive effect on the spray combustion characteristics of alcohol-diesel blended fuels. The combustion pressures of alcohol-diesel blended fuels were lower than that of pure diesel due to lower heating values, and this difference became more obvious under higher ambient temperature. Alcohols blended into diesel fuel could improve the combustion efficiency. The ignition delay times (IDTs) of alcohol-diesel blended fuels decreased with increased carbon chain lengths of alcohols under low ambient temperature conditions. However, this trend was reversed with increasing ambient temperature, and the IDTs of alcohol/diesel blends became lower than that of pure diesel at higher ambient temperatures. Propanol or hexanol blended into diesel could significantly inhibit the generation of soot at low ambient temperatures, but other alcohols promoted it. Interestingly, all alcohol-diesel blend fuels inhibited the generation of soot under high ambient temperature conditions, and propanol was the most significant.

Operational characteristics of oscillating heat pipe at sub-zero temperature for cold energy storage

This study first suggested a natural cold energy storage based on oscillating heat pipe (OHP) and presented the experimental investigation on the operational characteristics of the OHP at sub-zero temperatures for cold energy production. In the present experiment, the OHP was manufactured using a copper meandering tube with inner diameter of 2 mm and 5-turns, and R-134a was used as the working fluid. The manufactured OHP-based cold energy storage module was put into the temperature-controlled cooling chamber and the cold energy storage performance was evaluated at various low ambient temperature conditions. The thermal resistance of the OHP had a minimum value of 0.18–0.23 K/W regardless of the ambient temperature change. However, the lower ambient temperature caused significant thermal resistance decrease at water-ice temperature of 0 °C and the minimum thermal resistance was 1.18 K/W at the lowest ambient temperature of -20 °C. When the cold energy was stored in the final form of ice, the present OHP module could store the total maximum amount of cold heat of 2,784 kJ, consisting of sensible heat of 662 kJ and latent heat of 2,345 kJ at the lowest ambient temperature of -20 °C. It was concluded that the lower ambient temperature causes the stable and higher thermal performance of the OHP which increases the amount of stored cold heat by sensible heat and makes the OHP module possible to produce the cold heat more quickly.

Experimental Study on the Frosting and Heat Transfer Performance of Three Rows of Variable Pitch Finned Coils

Abstract The frost mass in different rows of the outdoor heat exchanger of an air source heat pump is uneven when the ambient temperature is below 0 °C, which leads to a shorter defrosting interval and reduces the heating capacity of the heat pump. In order to improve the problem of uneven frost along the airflow direction in the evaporator, an experimental study is conducted on the frosting and heat transfer performance of three rows of variable pitch (VP) finned coils in different fin pitch combinations under low ambient temperature conditions. The results show that the frosting time of the sample with VP of 5*3*2 is extended by 46.67%, the frost mass per unit of bare tube surface area is reduced by 14.54%, and the average heat transfer power per unit mass and volume is increased by 37.64% and 33.56%, respectively.

Study of microgeometry of contact surfaces of powder coatings and steel contact surfaces under sliding friction in cold conditions

This article discusses the changes in the operating temperature of friction and the patterns of formation of the microgeometry of the contact surfaces of modified powder coatings and steel counterbodies during sliding friction under conditions of low climatic ambient temperatures. Comparative studies of the roughness parameters of friction surfaces “powder coating — steel counterbody” during wear tests at room and low climatic temperatures are given.

Defrosting dynamic behaviors of a propane air conditioner and its rotary compressor under low ambient temperature conditions

Due to the great potential of propane in environmental protection and thermodynamic properties, the promotion of propane room air conditioner will make a great contribution to environmental protection. The defrosting behaviors of propane room air conditioner system and rotary compressor were studied by experiments in this paper. Experimental analysis redefines and distinguishes defrosting time and frost melting time, and their relationship with condition temperature is different. The experimental results show that the lower the ambient temperature was, the longer the defrosting time was, but the shorter the frost melting time on the outdoor heat exchanger was. The state of the oil sump and foam layer in propane rotary compressor using Mineral Oil (MO) was recorded and the variation reason for liquid level was analysed during defrosting. Combined with the change of viscosity and liquid level of the oil sump, the experimental results show that the compressor has abnormal oil discharged, and the lower the ambient temperature was, the larger the oil discharge was under three conditions during defrosting process. The total mass of the mixture in the oil sump under the three working conditions is 275.18 g, 289.10 g and 276.41 g, the lubricating oil is 202.97 g, 200.23 g and 189.71 g, and the refrigerant mass is 72.21 g, 88.87 g and 86.71 g respectively when the defrosting process ended. This study will provide an experimental basis of the reliability study and application of propane air conditioners.

A study of a low pressure ratio vapor injection scroll compressor for electric vehicles under low ambient conditions

Vapor injection technology is a promising method to improve the performance of heat pump systems on electric vehicles. In this work, an experimental study on a low pressure ratio vapor injection scroll compressor was carried out under low ambient temperature conditions. Comparative experiments were conducted for various injection pressures, compressor speeds, and evaporation temperatures. The typical flow and heat transfer characteristics were obtained and analyzed, especially the compressor volumetric efficiency and compressor efficiency. The results showed that the heating performance was improved and discharge temperature was reduced with the increasing injection pressure at a higher compressor speed and a lower evaporation temperature. The volumetric efficiency and compressor efficiency both decreased with the increasing injection pressure. The maximum heating capacity could increase by 16% and the maximum discharge temperature reduction would reach 8 °C for the vapor injection process compared with non-injection process under the evaporation temperature of -22 °C. Furthermore, a semi-empirical model with correlations of volumetric efficiency and compressor efficiency was developed. The results were validated and showed a reasonable agreement with experimental results.

Spray and combustion characteristics of butanol-diesel and hexanol-diesel blends under high-altitude conditions

Due to their excellent fuel properties, butanol and hexanol are considered as two promising renewable fuels for diesel engine applications. The objective of this work is to investigate and compare the spray and combustion characteristics of butanol-diesel and hexanol-diesel blends under high-altitude conditions. A constant volume combustion chamber was used to provide the desired environment. Experiments were conducted under various ambient temperatures ranging from 800 K to 1200 K and various ambient densities ranging from 11 kg/m3 to 15 kg/m3. Fuel blends with 20% blend ratio (B20 and H20) and neat diesel were used as test fuels. The spray and combustion characteristics of the test fuels were analyzed using measured liquid penetration length, ignition delay, combustion duration, flame lift-off length (FLoL), and spatially integrated natural luminosity (SINL). Results revealed that B20 and H20 had shorter liquid penetration length than diesel under low ambient temperature and density conditions due to their higher volatility. The blended fuels showed significantly longer ignition delay than diesel under low ambient temperature conditions, but it could be overcome by increasing the ambient temperature. Adding butanol and hexanol into diesel both extended FLoL especially under low ambient temperature and density conditions, but the effects of hexanol was less dramatic compared to butanol due to its lower latent heat of vaporization and higher chemical reactivity. Both B20 and H20 showed lower SINL than diesel under any conditions, indicating their ability of reducing soot emissions. However, no apparent differences could be found between these two fuels until the ambient temperature dropped to 800 K.

Investigation of Operation of Coil-Flow Steam Generator of Serpentine Type in Conditions of Low Ambient Temperatures

Stationary and mobile steam generators are widely used in low ambient temperatures, for example in areas described by the Köppen world map as subarctic. Such equipment is often used in oil and gas fields. At the moment, the existing standard boiler plants are outdated. The purpose of this work is an experimental study of a coiled-type direct-flow steam generator developed by the authors in the winter period at low ambient temperatures. The tasks to be solved to achieve the goal are associated with obtaining experimental data at different operating modes of the installation, their processing and the development of empirical coefficients of gas movement inside the coaxial cylinders of the steam generator. In addition, another task is to develop a theoretical basis for the obtained experimental data. Based on the results of the work, the dependences of pressure and temperature on fuel consumption in various modes were obtained. Statistical analysis was applied to the data obtained. The authors have developed equations for calculating the convective part in the process of radiant-convective heat transfer in coaxial gas ducts, taking into account the design features of a once-through coil-type steam generator. Finally, promising directions for further improving the efficiency of steam generators of this type are proposed.

Electrical and thermal performance comparison between PVT-ST and PV-ST systems

Both photovoltaic (PV), and photovoltaic thermal (PVT) are technologies that use solar energy for power output. Combining them with solar thermal (ST) can enable the generation of electricity as well as high-temperature hot water. In this study, The heat transfer model was established to investigate the thermal, electrical and overall performance of the two systems, which one is a separately operating PV and ST system (i.e. PV-ST system) and the other is a PVT and ST system in series (i.e. PVT-ST). By comparing the electrical, thermal and overall performance of the two combinations under different solar radiation, ambient temperature and inlet temperature, the application range, advantages and disadvantages of the two combinations are given, which can provide guidance for the design of higher electrical and thermal output system. The comparative study of the two systems shows that the electrical efficiency, thermal efficiency and primary energy saving efficiency of the PV-ST system are higher than that of the PVT-ST system under the conditions of low ambient temperature and solar radiation. This makes the PV-ST system have a wider application prospect in such low temperature and weak radiation environment conditions. However, under the environmental conditions of higher ambient temperature and stronger solar radiation intensity, the electro-thermal efficiency and primary energy saving efficiency of PVT-ST system will be higher than that of PV-ST system. The PVT-ST system will be widely used in high temperature and strong radiation conditions.

Performance evaluation of off-grid solar chemical heat pump for cooling/heating

Facing the current energy situation, the world’s energy supply and consumption is developing in diversification, cleanliness, and efficiency. Chemical heat pump (CHP) systems using calcium sulfate have been well studied in recent decades. However, systematic research based on simulations and experiments of coupling the off-grid CHP system and solar thermal system for cooling and heating (Solar Chemical Heat Pump: SCHP) is still insufficient. In this study, a simplified simulation model has been built for heat/mass transfer analysis of the SCHP heat storing step using the finite difference method (FDM) in order to make SCHP design easily. Simulated results were compared with previous experimental data to ensure feasibility. The effects of heat-transfer fluid type, heat exchanger structure, solar irradiance, and ambient air temperature on the chemical heat storage performance of the SCHP system were examined by the simulation. This study quantified reactant temperature, dehydration conversion, and COPs. Using the low-viscosity heat-transfer fluid, increasing the exchanger heat and the solar collector area can obtain a higher COP even under low solar irradiance and low ambient air temperature conditions.