Compression Heat Research Articles

Performance analysis and additive screening of a liquid carbon dioxide mixture energy storage system coupled with a coal-fired power plant

Liquid carbon dioxide (CO2) energy storage is a promising technology for balancing grid supply and demand, but liquefaction in high temperature environments is a substantial dilemma. In this study, a novel liquid CO2 mixture energy storage system coupled with a coal-fired power plant is proposed to broaden the liquefiable ambient temperature range, where condensate and feedwater without phase change are used for compression heat recovery and CO2 mixture heating. Firstly, eight refrigerant additives are selected to blend with CO2; then, comparative performance analysis between CO2 mixtures and pure CO2 is conducted, followed by parametric studies; finally, a double-layer decision-making framework based on multi-objective optimization is adopted for additive screening across various ambient temperatures. The results show that a moderate amount of additive favors system safety but degrades thermodynamics and economy, well at high temperatures the performance of CO2 mixture improves and absolutely outperforms pure CO2. CO2/difluoromethane (CO2/R32) reduces the system operating pressure by 20.95 % over pure CO2 at ambient temperature of 298.15 K, accompanied by only 5.78 %, 0.76 % and 9.92 % deterioration in round-trip efficiency, energy storage density and levelized cost of electricity, and exhibits optimal adaptation to varying additive mass fraction. The working fluid unit cost and heat exchangers are the main factors causing these variances. In response to environmental changes, CO2/propylene (CO2/R1270) and CO2/R32 emerge as the two most adaptable mixtures, with the former preferred at low ambient temperatures (288.15–293.15 K) and the latter at high ambient temperatures (298.15–308.15 K). Among all additives, R600 and R161 consistently rank lowest in priority regardless of ambient temperature.

Compression heat self-utilisation systems with cascade deep dehumidification for industrial compressor energy saving

Gas compressors are major energy consumers in many modern industries, including energy and chemical production. However, they face challenges of high wasted energy in the form of low-grade compression heat, and the increased power input and blade corrosion due to high-humidity suction gas. This study proposes a novel compression heat self-utilisation system that integrates liquid desiccant-based cascade dehumidification with absorption refrigeration to achieve efficient recovery of compression heat and deep dehumidification of suction gas. For the core module, a test bench for the cascade dehumidification unit is constructed to validate its deep dehumidification performance under high gas-desiccant flow ratios. Based on the validated numerical model, the feasibility of applying the proposed system to a three-stage compression process in air separation systems is evaluated in terms of water vapour condensation, energy savings, and economic and environmental benefits. Results show that deep dehumidification is achieved experimentally, with the humidity ratio reduced from 20.00 g/kg to below 6.00 g/kg, thus preventing water vapour condensation even when the gas-desiccant flow ratio reaches 9.65. when applied to a typical air separation plant, the compression heat self-utilisation system achieves an energy saving rate of compression work of approximately 4.10%. The payback period is as short as 3.21 years, with potential carbon emission reductions of up to 4610 ton/year in tropical regions. The work not only offers data support for liquid desiccant-based deep dehumidification systems but also lays a foundation for its energy-saving application in large-scale industrial compressor trains.

Evaluation and Improvement of the Flexibility of Biomass Blended Burning Units in a Virtual Power Plant

Aiming at the problems of small thermal power units and biomass mixed combustion units with small generation loads and insufficient primary frequency modulation capability, which cannot be connected to the virtual power plant, this paper adopts a variety of flexibility retrofit methods for the units and explores the peak load capability of the units. Then, multiple units are coupled, and the unit coupling scheme with better economy and environmental protection is screened using comprehensive evaluation indexes. While evaluating the peaking load space of multiple unit coupling, the units’ primary frequency regulation capability and new energy consumption capability are improved. According to the calculation results, the low-pressure cylinder zero-output retrofit has the largest peaking potential among different technical paths, in which unit #3 has 27.55 MW of peaking space. The compression heat pump decoupling retrofit has the best economy, in which the daily profit of unit #3 increases from 0.93 to 1.02 million CNY with an increase of 0.09 million CNY. After the unit has been retrofitted with steam extraction, the three units can be coupled to meet the national feed-in standards. The multiple unit coupling can accommodate up to 203.44 MW of other energy sources while meeting the standard.

Thermodynamic analysis and optimization of the semi-closed oxy-combustion supercritical CO2 power cycle with blending hydrogen into the natural gas/syngas

The semi-closed oxy-combustion supercritical CO2 (sCO2) cycle is regarded as a promising power cycle featuring the combined advantages of high efficiency and nearly 100 % CO2 capture. Blending hydrogen into fuels has attracted significant attention in the research of gas turbine and internal combustion engine power generation as it contributes to economy-wide decarbonization and bolsters the broader energy sources, especially green hydrogen. The present work carried out the thermodynamic analysis of a semi-closed sCO2 cycle to investigate how the hydrogen content in natural gas or syngas affects the operating parameters and performance. The results show that the energy efficiency decreases with increasing hydrogen content in natural gas whereas the maximum efficiency exists for the syngas with 60 % H2. The key factor is the increasing H2O content in the flue gas caused by adding hydrogen in fuels enhances both the heat-work conversion in the turbine and the heat transfer in the regenerator. The optimal turbine inlet temperature is around 1150 °C, which is the same as the basic cycle. The optimal turbine inlet pressure for natural gas and natural gas with 40 % H2 is both 28 MPa, while the optimal turbine inlet pressure decreases with hydrogen content in syngas. For all the fuels, the optimal turbine outlet pressure becomes lower for the higher hydrogen content. The maximum efficiencies obtained for natural gas, natural gas with 40 % H2, and syngas with 60 % H2 are 56.08 %, 55.52 % and 56.15 %. Finally, it is found that the recompression cycle and the multiple combustor arrangement (reheat cycle) cannot be the effective configuration for the semi-closed oxy-combustion sCO2 power cycle. The heat from the sCO2 recompression conflicts with the compression heat of the ASU, reducing the energy efficiency by 0.27 %, 0.34 %, and 1.23 % for the three fuels, respectively. The limitation of the regenerator allowable temperature requires the reheat cycle to either reduce the turbine inlet temperature or reduce the turbine outlet pressure, decreasing the energy efficiency for natural gas by 1.83 % and 6.38 % due to the reduced turbine output power or significantly increasing compression power.

Recent Advances in Ejector-Enhanced Vapor Compression Heat Pump and Refrigeration Systems—A Review

Recent Advances in Ejector-Enhanced Vapor Compression Heat Pump and Refrigeration Systems—A Review

Diesel Control Strategy in Dual-Fuel Engine

Dual-fuel engines are capable of operating using a mixture of two different fuels. Generally, the primary fuel is a gaseous fuel (CNG, LPG, Hydrogen), while diesel is used as a pilot ignition source, i.e. functioning on heat of compression and not a spark plug. There have been a few dual-fuel engine strategies to control the speed of the engine based on varying loads, but these require the addition of complex mechanisms to the existing systems. This paper discloses a methodology for controlling the diesel quantity during dual-fuel operation. According to the engine speed and accelerator pedal position, a gaseous fuel injector in the system injects a predetermined quantity of gaseous fuel. Based on the regulated quantity of gaseous fuel, the intake manifold creates an air-gas fuel mixture. The air-gaseous fuel mixture and a metered quantity of diesel are delivered to the combustion chamber via the intake manifold, fuel injection pump, and other components. The governor and fuel injection pump work together to measure the quantity of diesel according to speed and load conditions. During dual-fuel operation, the governor regulates engine speed by controlling the quantity of diesel delivered to the engine based on speed and load conditions. Keywords: Dual-fuel, Control Strategies, Off-Road Engines, Combustion, Emissions, Diesel Replacement

Thermal analysis of a compression heat pump-assisted solar still for Caspian regions of Kazakhstan

Thermal analysis of a compression heat pump-assisted solar still for Caspian regions of Kazakhstan

Enhancing mechanical properties of Al-Zn-Mg-Cu alloys: The impact of high strain rate compression and subsequent heat treatment on microstructural evolution

This study presents a comprehensive examination of the Al-Zn-Mg-Cu alloy, focusing on the effects of high strain rate dynamic compression and heat treatment on its performance. The research aimed to understand the underlying microstructural mechanisms contributing to the enhanced mechanical properties of the alloy, which is critical for applications in aerospace and automotive industries. High strain rate dynamic compression tests, conducted at 3.0 × 1000 s−1, revealed a significant increase in both yield strength and ultimate compression strength of the Al alloy, which was accompanied by a increase in hardness. Post-compression microstructural analyses utilizing transmission electron microscopy and scanning- transmission electron microscopy indicated that significant changes occurred during testing. Key findings include the development of micro shear bands, alterations in the size and distribution of precipitates, and the emergence of new plate-like precipitate formations. Subsequent T6 heat treatment led to further microstructural evolution, particularly impacting the volume fraction and size of η' precipitates. The study highlights the crucial role of dislocation-precipitate interactions, where dislocations are effectively pinned by precipitates, thereby contributing significantly to the material's strength. These results demonstrate the potential of high strain rate dynamic compression coupled with heat treatment as a method to significantly enhance the mechanical properties of Al-Zn-Mg-Cu alloys. Overall, the findings provide valuable insights into the optimization of this alloy for high-performance applications, emphasizing the importance of controlled microstructural engineering.

Design, analysis and optimisation of a novel adiabatic-isothermal CAES system with coupled stepped phase change energy storage unit

Compressed air energy storage (CAES) technology is one of the important technologies to address the instability of renewable energy sources. To further make full use of the system heat of compression and reduce the problem of energy grade dissipation inside the accumulator, this paper proposes a novel CAES system coupled with a graded phase change thermal energy storage unit (SP-TES) and near isothermal compression of a liquid piston. A combination of adiabatic compression and near-isothermal compression is used to reduce the generation of compression heat, while the graded storage of compression heat through SP-TES reduces the reduction of energy grade. The design of the proposed SP-TES is carried out through the binary eutectic theory, and a one-dimensional transient entransy dissipation model, a thermodynamic model and an economic model are developed to analyze the 4E performance (energy, exergy, entransy, and exergoeconomic) of the proposed SP-TES. Further, the effect patterns of key node parameters on the proposed system are investigated by developing system related models and the system is optimized with multiple objectives from the point of view of economic and thermodynamic performances. The results show that the 4E performance of SP-TES is in between that of high and low temperature single-stage accumulators, and that SP-TES combines the advantages of higher economic performance of single-stage low-temperature accumulators and higher thermal performance of single-stage high-temperature accumulators. The SP-TES with a temperature range of 350 K-451 K is gradually stabilized with the number of charging and discharging, and the efficiency of the system in the stabilization stage is 73.24 %. The SP-TES with a temperature range of 407 K-440 K has the best economic performance with exergoeconomic parameter of 360.05 kJ/USD. The thermal storage performance and power output of the SP-TES decreases with the increase in the number of stages. The multi-objective optimization results show that the new CAES system efficiency is 63.03 % and the design is optimal when the unit energy cost is 0.2217 $/kWh.

Effect of thermoelectric subcooling on COP and energy consumption of a propane heat pump

The building sector has an important impact on the environment, being responsible for 30 % of the total greenhouse gas emissions. Knowing that the energy consumption devoted to HVAC systems accounts for 50 % of the total energy consumption of buildings, it is paramount to develop environmentally friendly technologies able to provide green space heating to the building sector. To that purpose, this manuscript presents a computational study on propane vapor compression heat pumps which include thermoelectric subcooling to boost their operation. The combination of these technologies has been proven in the past to be very beneficial for refrigeration systems and this study concludes for the first time that propane heat pumps can highly benefit from thermoelectric subcooling. The widely conducted research includes the following parameters: ambient temperatures from −20 to 15 °C, voltage supplies to the thermoelectric modules from 0.5 to 10 VDC, number of thermoelectric subcooling blocks from 1 to 8 and two water inlet temperatures, 40 and 55 °C to study their influence on heating capacity, compressor and thermoelectric power consumptions, subcooling degree, propane mass flow, compressor capacity, COP, energy consumption and SCOP of the combined heat pump. The obtained results are very conclusive, COP enhancements up to 12.29 % are achieved when a thermoelectric subcooler with 16 modules is included in a propane heat pump already provided with an internal heat exchanger for an ambient temperature of −20 °C and a water inlet temperature of 55 °C. Additionally, improvements in Seasonal COP up to 9.98 % are achieved if the above-mentioned technologies integration between a vapor compression heat pump and a thermoelectric subcooler substitutes a conventional propane heat pump with an internal heat exchanger for space heating a single-story two-family house.

Comparative analysis of solar-powered heat pump systems for decarbonization of process heating and cooling applications: Case of milk pasteurization

The dairy processing industry is heavily dependent on fossil fuels for heating applications at different temperature levels and contributes to greenhouse gas (GHG) emissions. Besides, process cooling, product refrigeration, and storage are mainly reliant on vapour compression refrigeration technologies. This study investigates the performance of solar photovoltaic (PV) and solar thermal (ST) powered heat pumping technologies for simultaneous heating (at 75 °C) and cooling (at 2 °C) applications in a selected typical thermal process in a dairy processing plant (milk pasteurization). An ammonia/water absorption heat pump (AHP) and a two-stage R717 vapour compression heat pump with an open intercooler (VCHP) are used in the present study. The VCHP and AHP are powered by market-available solar PV modules and ST collector technologies. The electrical COP and exergy COP of the VCHP were about 4.710 and 0.535, respectively. The AHP operates at a thermal COP and exergy COP of 0.979 and 0.301 when the driving hot water and absorber cooling water inlet temperatures are 160 °C and 30 °C. The grid-connected PV-VCHP system and ST-AHP with natural gas-fired auxiliary heater operate at an annual solar fraction of 0.533 and 0.572, respectively, for a typical weather condition in Barcelona (Spain).

Towards Direct Numerical Simulations of Reactivity-Controlled Compression Ignition Engine Using n-Octanol/Ethanol Fuel Blends

AbstractThe results of a two-dimensional direct numerical simulation of a lean n-octanol-ethanol fuel blend under Reactivity Controlled Compression Ignition (RCCI) conditions are presented in this paper. Stratified temperature and high reactivity fuel fields of Gaussian, bi-modal, and log-normal distributions are studied for uncorrelated and correlated scenarios. The pimple loop is made to run twice to achieve compression heating. A chemical mechanism with 171 species and 734 reactions was developed to capture autoignition characteristics and flame propagation reasonably well. Diagnosing techniques published in the literature are used to determine whether the flame fronts are spontaneously propagating or not. As reported previously for other fuel blends under RCCI conditions, both deflagration and spontaneous ignition flame fronts are observed to co-exist. Gaussian, bi-modal, and log-normal fields respectively move towards a spontaneously igniting mode. Correlating temperature and high reactivity fuel fields not only makes combustion more spontaneously igniting but also more premixed. The analysis reveals the sensitivity of the DNS results with respect to the initial conditions which accordingly should be chosen with care.

Performance Analysis of an Ejector-Enhanced Heat Pump System for Low-Temperature Waste Heat Recovery Using UHVDC Converter Valves

This article proposes a heating method based on heat pump technology to address the large amount of low-grade waste heat generated by a certain type of ultra-high voltage direct current (UHVDC) converter valve. Thermal performance calculations for two systems, a basic vapor compression heat pump system (BVCHPS) based on thermal expansion valve throttling and an ejector-enhanced heat pump system (EEHPS) are analyzed. The research results show that the EEHPS exhibits superior COP and exergy efficiency when generating hot water above 80 °C using a heat source below 50 °C. Additionally, mathematical modeling analysis identifies optimal structural parameters such as nozzle throat diameter, throat area ratio, and nozzle outlet diameter for the ejector in its design state. The low-temperature waste heat recovered from the UHVDC converter valves can be further used in engineering applications such as heating, refrigeration, seawater desalination, and sewage treatment.

Vapor absorption, compression and cascade heat pumps for carbon capture plants: Multi-criteria analysis and techno-economic assessment with different working fluids

Post-combustion CO2 capture plants include high costs that are associated with intense energy consumption. Heat pumps can recover heat from the flue gas and other sources in the CO2 emitting plant and upgrade it to reduce the use of fossil-based steam. The few studies regarding heat pumps in CO2 capture systems include arbitrary selection among limited working fluids, without optimizing the underlying cycle. This work implements a multi-criteria investigation of the performance of 20 working fluids for vapor compression heat pumps (VCHP) and 21 refrigerant/absorber pairs for absorption heat transformers (AHT), considering the operating optimization of each cycle for each fluid. A cascade cycle combining the two configurations (AHT-VCHP) is proposed, using the optimum fluids cyclopentane and water/potassium nitrate identified for the VCHP and the AHT. The annual operating costs are higher than the annualized capital expenditures. The proposed fluids outperform the conventional options. The VCHP with cyclopentane covers 58 % (2.68 GJ/t CO2) of the reboiler duty with recovered heat and exhibits the lowest cost of $33.8/tCO2. This is $18.7/tCO2 lower than the cost of the conventional direct contact cooling unit that is used in capture plants. The VCHP remains competitive for up to 57 % increase in the electricity price.

Performance Analysis of a New Cogeneration System with Efficient Utilization of Waste Heat Resources and Energy Conversion Capabilities

This paper proposes a new type of cogeneration system coupled by Organic Rankine Cycle (ORC), Absorption Heat Pump (AHP), and Compression Heat Pump (CHP). The new system can meet the needs of different scenarios. Simulations were conducted to analyze the effect of different factors on the parameters and performance indexes of the new system and compared with the system of ORC, AHP, CHP. The results showed that the factors of flue gas temperature at evaporator1 outlet and CHP have the greatest effect on the parameters and performance indexes of the Organic Absorption-Compression Coupling Heat Power (OACCHP) system, and the effect of AHP on the parameters and performance indexes of the OACCHP system can be ignored. In the given range of flue gas temperature at evaporator1 outlet, the heat production of total and net work is reduced by 1.17% and 33.33%, respectively. In a given range of CHP evaporation temperature, the heat production of the total is reduced by 5.74%. In the given range of the outlet temperature of the gas cooler, the heat production of the total is increased by 10.58%. In a given working condition, compared with the single ORC system, the single CHP system, and the single AHP system, the thermal efficiency, COP, and energy earning rate of the OACCHP system are increased by up to 755.49%, 59.8%, 5.8%, respectively. Finally, the optimal operating conditions for different scenarios were determined.

Mechanical and thermal property analysis and optimization design of hybrid lattice structure based on triply periodic minimal surfaces

Hybrid lattice structures have significant potential in engineering applications owing to their outstanding mechanical and thermal properties. In this paper, an innovative design strategy for hybrid lattice structures is proposed involving the integration of primitive surfaces with truss lattice configurations. This approach allows for the full exploitation of the mechanical and thermal properties inherent in each component, leading to the development of new hybrid lattice structures. Firstly, the quasi-static compression and fluid-solid coupling heat conduction analysis of the hybrid lattice structure is carried out by numerical simulation. Then the effects of surface wall thickness, and truss diameter on the mechanical and thermal properties of the lattice structure are discussed. Additionally, the mechanical performance of the array hybrid lattice structure is evaluated through quasi-static compression experiments. These experimental results are compared with those from the numerical simulations, validating the accuracy of the simulation model. Finally, a multi-objective optimization model targeting specific elastic modulus and heat dissipation performance (indicated by Nusselt number) is established, in response to the demand of aviation and aerospace industries for lightweight, load-bearing, and heat-dissipating multi-functional integrated lattice design. The non-dominated sorting genetic algorithm is utilized to solve the optimal model, and the distribution of feasible solutions and Pareto front are obtained. The results show that the interpenetrating hybrid lattice structure has a greater improvement in mechanical and thermal properties than the primitive surfaces. This research holds significant implications for the design of multi-performance hybrid lattice structures.

Performance analysis and configuration method optimization of AA-CAES-based air storage tanks

To improve the performance of the compressed air energy storage (CAES) system, flow and heat transfer in different air storage tank (AST) configurations are investigated using numerical simulations after the numerical model has been experimentally validated. System performance for different AST placement methods is analyzed through numerical simulations integrated with the thermodynamic model of advanced adiabatic compressed air energy storage (AA-CAES). An in-depth study examines the impact of key system parameters on system performance with different AST configurations. Based on these analyses, the AA-CAES system with a constant volume of AST is optimized. The results indicate that horizontal placement of the AST improves heat transfer capability within the same working pressure range but results in slightly lower energy storage efficiency, achieving 64.61% compared to 65.50% for vertical placement. However, horizontal placement offers higher energy storage density, achieving 3.54 kW h/m3 under specific conditions, compared to 3.14 kW h/m3 for vertical placement. As the energy storage flow rate increases, exceeding the critical flow rate significantly improves heat transfer in vertically placed ASTs, thus narrowing the energy storage density gap between configurations. Increased turbine efficiency, additional external heat sources, and further utilization of compression heat provide more significant performance improvements for the AA-CAES with the AST placed horizontally compared to vertically. Compared to the AA-CAES with vertically placed ASTs, the configuration of the ASTs is optimized to enhance the electrical output of the AA-CAES by 76.4 MW h and reduce the input by 78.9 MW h at a storage flow rate of 0.5 kg/s.

Temperature-reducing shocks in optically thin radiative MHD—Analytical and numerical results

Shocks are often invoked as heating mechanisms in astrophysical systems, with both adiabatic compression and dissipative heating that leading to the increase in temperature. While shocks are reasonably well understood for ideal magnetohydrodynamic (MHD) systems, in many astrophysical plasmas, radiation is an important phenomena, which can allow energy to leave the system. As such, energy becomes non-conservative, which can fundamentally change the behavior of shocks. The energy emitted through optically thin radiation post-shock can exceed the thermal energy increase, resulting in shocks that reduce the temperature of the medium, i.e., cooling shocks that have a net decrease in temperature across the interface. In this paper, semi-analytical solutions for radiative shocks are derived to demonstrate that both cooling (temperature decreasing) and heating (temperature increasing) shock solutions are possible across the whole temperature range in optically thin radiative MHD. Numerical simulations of magnetic reconnection for solar-like temperatures and plasma-β with optically thin radiative losses also yield both heating and cooling shocks in roughly equal abundances. The detected cooling shocks feature a significantly lower pressure jump across the shock than their heating counterparts. The compression at the shock front leads to locally enhanced radiative losses, resulting in significant cooling within a few grid cells in the upstream and downstream directions. The presence of temperature-reducing (cooling) shocks is critical in determining the thermal evolution, and heating or cooling, across a wealth of radiative astrophysical plasmas including magnetic reconnection in the solar corona.

Thermodynamic Analysis of Modelled and Simulated Heat Pump Drying System Using Azeotropic Mixture of Organic Working Fluids

It is well-known that the choice of working fluid significantly impacts the performance, cost estimation, and environmental effect of a heat pump. In this study, the performance of pure working fluids (dimethyl ether and ethanol) was compared with that of azeotropic mixture working fluids used in vapor compression heat pumps. The study also examined the effect of compression ratio and different compressor models on the performance of a vapor compression heat pump for heating processes and drying tomato slices at an air temperature of 40 °C and an air flow rate of 200 kg/hr. Using ethanol as the working fluid at evaporating and condensing temperatures of 10 °C and 15 °C, respectively, the specific moisture extraction rate was 0.2112 kg/kWh, and the Carnot coefficient of performance was 57.60. The study demonstrated a COP improvement of more than 15% for azeotropic mixtures compared to a pure working fluid. The maximum overall energy efficiency and exergy efficiency, achieved with Mixture II, were 5.68% and 27.32%, respectively. As suction pressure increased while maintaining constant discharge pressure, the compression ratio and work done by the compressor decreased, while the system's overall energy efficiency, and overall exergy efficiency improved under the given conditions.

Heat transfer efficiency of thermoelastic shaped phase change materials enhanced by pressure

As the melt front gradually moves away from the heat source during the heat transfer process, phase change materials (PCMs) inevitably cause the heat transfer efficiency to decrease with heat transfer time, especially for a shaped PCM (SPCM). In this study, it is proposed to compress the SPCM phase-changed region by pressure to reduce the thermal resistance between the heat source and the phase transition interface, thus slowing down the rate of heat transfer efficiency decrease. We established a one-dimensional semi-infinite compression heat transfer theoretical model and prepared a thermoelastic SPCM (TESPCM) for compression heat transfer experiments, which verified the reliability of the theoretical model. Experimental demonstrate that the pressure enhancement contributes to the heat transfer efficiency and becomes more pronounced with heat transfer time. The pressure-enhanced heat transfer was analyzed by the theoretical model with the following conclusions: pressure enhancement not only improves the latent heat utilization, heat transfer cut-off time, and maximum tolerable heat flux of TESPCM but also increases the specific energy and specific power of TESPCM. The effect of pressure on heat transfer efficiency tends to accelerate with increasing compression. Furthermore, the improvement of heat transfer by pressure enhancement becomes more significant with the increase of thermal conductivity, but the effect of latent heat on temperature control decreases with the increase of compression.