Coolant Flow Direction Research Articles

Thermal management characteristics of a novel cylindrical lithium-ion battery module using liquid cooling, phase change materials, and heat pipes

To improve the thermal performance of large cylindrical lithium-ion batteries at high discharge rates while considering economy, a novel battery thermal management system (BTMS) combining a cooling plate, U-shaped heat pipes, and phase-change material (PCM) is proposed for 21700-type batteries. The effects of variables such as the contact angle between a corrugated aluminum plate (CAP) and the battery, the coolant flow direction in the cooling plate, the type of PCM, and the coolant flow velocity on the thermal characteristics of the batteries in the BTMS are investigated, and the advantages of this coupling cooling strategy relative to a single active cooling or a single active and passive cooling combination strategy are discussed. The results indicate that a corrugated aluminum plate is beneficial for significantly improving the temperature uniformity of the batteries. A staggered flow-direction scheme of adjacent channels significantly improves the temperature uniformity relative to the other inlet flow modes. The thermal performance of the batteries in the BTMS using paraffin and expanded graphite (PA/ EG) is superior to that with pure paraffin or paraffin and copper foam (PCF), with temperature uniformity improved by 44.6 % and 16.0 %, respectively. Increasing the coolant flow velocity can decrease the maximum temperature of the batteries and significantly reduce the heat dissipation share of PCM, resulting in a decrease in the temperature uniformity of the batteries in the BTMS. Meanwhile, compared with pure liquid cooling or a combination of liquid cooling and heat pipe or PCM, the proposed coupling BTMS not only reduces the maximum temperature of the batteries and further improves the temperature uniformity, but also significantly reduces the operating energy consumption of the BTMS.

Numerical simulation for non-uniform PtCo catalyst degradation under different coolant conditions and its effect on PEMFC performance

Long-term non-uniform temperature, pressure and reaction rates distributions in the proton exchange membrane fuel cells (PEMFCs) may result in local catalyst degradation and deteriorate PEMFC performance. However, the problem of non-uniform catalyst degradation in PEMFC stacks is still incompletely understood due to the lacking of advanced experimental methods and three-dimensional (3D) catalyst aging models. In this study, a 3D PtCo alloy catalyst degradation model under voltage cycles is built and a 25 cm2 PEMFC stack containing realistic cathode, anode as well as coolant flow fields is used as the computational domain to analyze in detail the influence of non-uniform catalyst degradation on the performance of the PEMFC during different coolant conditions. The simulation results show that the non-uniform catalyst degradation is strongly dependent on the cathode flow field structure. It can alleviate non-uniform catalyst degradation as well as contribute to improving PEMFC performance when the coolant flow direction is the same as the air, and both coolant flow rate and inlet temperature also are found to significantly affect the local catalyst degradation behavior. Therefore, an optimally designed cathode flow fields structure as well as good water and heat management during operation can help to extend the service life of the PEMFC.

Design and optimization of an integrated liquid cooling thermal management system with a diamond-type channel

The ability of the battery thermal management system (BTMS) to dissipate heat significantly impacts the operating performance and the service life of lithium-ion batteries (LIBs). Designing an effective and efficient BTMS is difficult due to its multiple parameters. Therefore, an integrated liquid cooling plate with a diamond-type flow channel was designed in this paper. Firstly, two arrangement plans (plan 1 and plan 2) were designed to investigate the effect of coolant flow direction in the channel on the cooling performance of BTMS. The simulation results indicated that plan 2 (the shorter main flow channel serves as the outlet) could lead to better cooling performance. The maximum temperature (Tmax) and the volume-average temperature (VAT) of the battery pack for plan 2 were 313.33 K and 308.79 K, respectively. Then, the surrogate model was developed by the Latin Hypercube Sampling (LHS), and the six parameters (F, Hc, Wb, WL, WS, and WIO) of the BTMS were optimized using the multi-objective genetic algorithm (MOGA). The Tmax of the battery pack decreased from 313.33 K to 308.98 K, and the VAT decreased from 308.79 K to 306.06 K. The average pressure drop (Δp) between the inlets and outlets decreased significantly, from 1708.30 Pa to 520.10 Pa, up to 1188.20 Pa. It demonstrated that the BTMS achieved a balance between thermal performance and power consumption after optimization.

Effects of module size on the heat dissipation performance of the thermal management system for the battery module with cylindrical cells

Heat dissipation performances of battery thermal management systems (BTMSs) reported in the literature are usually evaluated based on the modules with different sizes. Yet, effects of module size on the cooling performance of liquid-cooling BTMS with mini-channel cold plates are still unclear, which makes the performance comparisons of BTMSs from different researches difficult. In this context, systematic numerical investigations are conducted in this work to analyze the thermal performances of cylindrical battery modules with rectangular array structures ranging from 4 × 8 to 4 × 16, 4 × 32 and 4 × 48. The results demonstrate that increased module size is unfavorable to the heat dissipation performance of BTMS, especially in terms of temperature uniformity. To ameliorate the heat dissipation performance of BTMS, extended studies are performed to analyze the thermal behaviors of BTMSs with different module sizes under various coolant velocities and flow direction arrangements. Special attention is paid to their cooling performances under low coolant velocities, which are commonly ignored in previous studies. Based on the results, merits and demerits of different optimization methods are elucidated via in-depth mechanism analyses. Importantly, ultra-low velocity (0.0025 ms-1 in this work) operation is found a promising method to ease the contradiction between the cooling performance and the increased energy consumption owing to the dramatically enhanced temperature uniformity. The results could provide useful guidance for the future design of BTMS in the cylindrical battery module.

Numerical study on heat transfer characteristics and performance evaluation of PEMFC based on multiphase electrochemical model coupled with cooling channel

In this study, a three-dimensional multi-phase proton exchange membrane fuel cell (PEMFC) electrochemical model coupled with a cooling channel (CC) was developed, to comprehensively analyze the heat transfer characteristics. The membrane temperature, index of uniform temperature (IUT), net power, and the Nusselt number were applied to evaluate the heat transfer performance. The results indicate that a smaller IUT value achieves better PEMFC performance at close temperatures, and the best performance is achieved with a coolant inlet temperature of 343.15 K. Although increasing the coolant flow velocity improves the cooling effect, thereby enhancing PEMFC performance. However, it inevitably leads to an increase in parasitic power, resulting in a decrease in the output power of the PEMFC system. Moreover, with the coolant temperature difference less than 6K, PEMFC shows better performance when the coolant flow direction is the same as O2. On the contrary, with the coolant temperature difference greater than 6K, the coolant flow direction should be the same as H2 to ensure good performance. In addition, with the temperature difference between the coolant inlet and outlet of 10K, 6K, and 3K, respectively, the surface Nusselt number of wavy CC is 5.46%, 8.92%, and 18.71% higher than the straight CC, respectively.

Performance analysis of liquid cooling battery thermal management system in different cooling cases

An efficient battery thermal management system can control the temperature of the battery module to improve overall performance. In this paper, different kinds of liquid cooling thermal management systems were designed for a battery module consisting of 12 prismatic LiFePO4 batteries. This paper used the computational fluid dynamics simulation as the main research tool and proposed a parameter to evaluate the performance of the cold plate in terms of both heat transfer and flow resistance, then the effect of cooling surface, number of inlets, and direction of coolant flow on the cooling effect were studied. It was found that the maximum temperature of the battery module could be controlled at 303.6 K and the maximum temperature difference between the batteries at 2.3 K when the cooling surface was Face A which was between the batteries, the number of coolant inlets was 3, and the coolant flow direction was arranged alternately. Finally, the study was conducted for different mass flow rates of a single inlet and different charging rates. It was found that the overall performance is optimal when the mass flow rate of a single inlet is 1.2 g/s and the designed thermal management system can meet the temperature requirement (the maximum temperature of the battery module is 313 K and the maximum temperature difference between the batteries is 5 K) of the battery module with charging rate below 3C.

Cross flow and distribution characteristics in automobile polymer electrolyte membrane fuel cells: A three-dimensional full-scale modeling study

Ultrahigh power density is of primary importance to next-generation polymer electrolyte membrane fuel cell (PEMFC) for commercialization, which demands an in-depth insight into the intricate multi-physics transports, particularly for large-size PEMFCs in automobile application. In this study, a three-dimensional two-phase model is extended to study an automobile fuel cell with the active area of 366.6 cm2, including full morphologies of metallic bipolar plate, flow field, and distribution zones for hydrogen, air and coolant flow. It is found that the cross flow inside membrane electrode assembly under the rib between two adjacent channels promoted by the distribution zone can effectively mitigate local oxygen starvation and water flooding, increasing PEMFC performance at high current density, in comparison with conventional single-channel or no-dot-matrix models. Theoretical analysis indicates that the dot matrix distribution zone induces a larger pressure gradient and hence the cross flow than the commonly-used channel & rib distribution zones. Additionally, the counter-flow configuration has a higher cell performance and more uniform current density than the co-flow one due to internal humidification. When the internal humidification effect is weak, for example, in anode/cathode gas co-flow configuration, coolant flow direction becomes more important for the temperature profile and prevent severe dehydration.

A Novel Active Cooling System for Internal Combustion Engine Using Shape Memory Alloy Based Thermostat.

Pollutants in exhaust gases and the high fuel consumption of internal combustion engines remain key issues in the automotive industry despite the emergence of electric vehicles. Engine overheating is a major cause of these problems. Traditionally, engine overheating was solved using electric pumps and cooling fans with electrically operated thermostats. This method can be applied using active cooling systems that are currently available on the market. However, the performance of this method is undermined by its delayed response time to activate the main valve of the thermostat and the dependence of the coolant flow direction control on the engine. This study proposes a novel active engine cooling system incorporating a shape memory alloy-based thermostat. After discussing the operating principles, the governing equations of motion were formulated and analyzed using COMSOL Multiphysics and MATLAB. The results show that the proposed method improved the response time required to change the coolant flow direction and led to a coolant temperature difference of 4.90 °C at 90 °C cooling conditions. This result indicates that the proposed system can be applied to existing internal combustion engines to enhance their performance in terms of reduced pollution and fuel consumption.

Effect of Liquid Cooling Structure of Confluence Channel on Thermal Performance of Lithium-Ion Batteries

AbstractIn this study, based on the liquid cooling method, a confluence channel structure is proposed, and the heat generation model in the discharge process of three-dimensional battery module is established. The effects of channel structure, inlet mass flowrate, and coolant flow direction on the heat generation of the battery module were studied by control variable method. Simulation results show that the confluence channel structure (e) shows good cooling effect on the battery temperature when controlling the 5 C discharge of the battery module. In addition, compared with the straight channel under the same working condition. In the discharge process of battery module, average temperature amplitude in battery module decreased by 17.3%, the inlet and outlet pressure is reduced by 16.47%, and the maximum temperature amplitude is reduced by 20.3%. Effectively improve temperature uniformity and reduce pressure drop. The problem of uneven temperature distribution caused by uneven velocity distribution of coolant in traditional straight channel is improved. At the same time, the design of the confluence structure accelerates the heat transfer of the channel plate and provides a new idea for the design of the cooling channel.

Experimental study of the thermal and power performances of a proton exchange membrane fuel cell stack affected by the coolant temperature

The optimization of thermal management (i.e. improving heat generation and dissipation) in a proton exchange membrane fuel cell (PEMFC) is a vital method to enhance its energy conversion efficiency. However, few studies have focused on the temperature uniformity in a massive PEMFC stack with a coolant flow channel. The aim of this study is to investigate the temperature characteristics and power performance affected by the inlet coolant condition in a massive PEMFC stack and establish a correlation of the temperature uniformity with the affected inlet coolant temperature for thermal management and power performance improvements in PEMFCs. The power density of a massive PEMFC stack with an active area of 326 cm2 (302 mm × 108 mm) of the designed stack cells with a coolant flow channel were experimentally evaluated, and the thermal characteristics were also analyzed using monitored temperatures inside specific stack cells by considering the different inlet coolant temperatures of 50 °C, 60 °C and 70 °C. It was found that the stack had the greatest power performance and minimum temperature increment of the inlet coolant at a temperature of 70 °C. The water phase transition heat transfer inside the stack was found to play a significant role in the thermal balance. The temperature distribution of the stack cell had a symmetrical convex curve from the center to the margin. The temperature gradient of the stack cell along the coolant flow direction was introduced to characterize the temperature uniformity. The linear development between the temperature gradient and the current density was determined adequately using a curve fitting. In addition, a lower fitting linear slope was obtained for a higher inlet coolant temperature, and this signified that a higher inlet coolant temperature could improve the temperature uniformity and power performance of PEMFCs.

Experimental investigation and decoupling of voltage losses distribution in proton exchange membrane fuel cells with a large active area

Understanding the local operating state and voltage loss distributions are of great importance to guide the performance improvement of proton exchange membrane fuel cells (PEMFCs). Especially when the active area is large, the uneven electrochemical reaction becomes more severe. However, the current research does not enable the simultaneous monitoring of the multiple operating states and decoupling of the voltage losses in the fuel cell. In this study, a method is established that can obtain the distribution of activation polarization, ohmic loss, and concentration polarization by analyzing the measured data of temperature, humidity, current density, and high-frequency resistance (HFR), which are obtained inside a 320 cm2 and 3 kW level fuel cell stack by using the multilayer printed circuit board (PCB) technology with designed isolation segments. The effects of humidity, operating temperature, and current density on the distributions of output voltage and voltage losses are discussed. Results show that the potential distribution on the surface of membrane exchange assembly (MEA) isn’t uniform in the large active area. The activation polarization gradually increases in the direction of coolant flow and the direction of the cathode flow and the ohmic loss and concentration polarization have opposite distribution characteristics. In addition, when the humidity rises to full humidity, the inhomogeneity of ohmic loss distribution becomes higher with three local increased areas of 20 mV and one decreased area of −20 mV. Higher temperatures can significantly reduce the flooding near the outlet of the active area. When the current density increases, the inhomogeneity of the ohmic loss distribution increases, and the concentration polarization near the cathode outlet of the active area become more severe.

Numerical research on lithium-ion battery thermal management utilizing a novel cobweb-like channel cooling plate exchanger

As a key component of a pure electric vehicle, the battery in an overheated state will have a direct impact on battery life and vehicle safety. To promote battery heat dissipation, a novel cobweb-like type (C-type) channel cooling plate with asymmetric inlet and outlet is designed. The C-type channel cooling plate is numerically simulated in two coolant flow directions (Ifd and IIfd), using the computational fluid dynamics software STAR-CCM+, and compared to the conventional serpentine type (S-type) channel. Meanwhile, the effects of three structural parameters (channel diameter, spacing, and cooling plate thickness) on maximum temperature and temperature difference of the C-type cooling plate, and pressure drop are investigated. Based on this, the C-type channel is optimized by orthogonal test. The results show that the C-type with IIfd coolant flow direction has a better heat dissipation effect on the battery module than the C-type with Ifd and S-type under the same conditions, and the maximum temperature and temperature difference are respectively reduced by 0.2% and 17.8%, while the pressure drop is increased by 17.3%. In addition, increasing channel diameter can obtain good battery temperature distribution and smaller pressure drop, while the increase of cooling plate thickness and channel spacing has a greater effect on the battery temperature difference compared to the change of maximum temperature. Finally, the results of the orthogonal tests show that the cooling effect is best when the diameter of the cobweb-like channel cooling plate is 7 mm, the thickness of the cooling plate is 12 mm, and the channel spacing is 16 mm, and the maximum temperature and temperature difference are reduced by 0.7% and 6.8%, respectively, and the pressure drop is reduced by 37.6% compared to the initial cobweb-like channel scheme. This offers a fresh perspective on cooling plate channel design in liquid-cooled battery thermal management.

Experimental investigation and numerical analysis on effects of swirling coolant on flow characteristics and film cooling performance

Modern gas turbines use combined internal and external cooling methods to achieve higher cooling effectiveness. The internal cooling methods, like internal serpentine channel cooling and impingement cooling, will introduce swirl to the coolant of film cooling. In this paper, film cooling effectiveness of one row and two rows of holes with and without swirling coolant was measured using pressure sensitive paint (PSP). Numerical simulations were performed to analyze the flow structures. Results show that swirl in coolant enhances film cooling effectiveness for one row of holes when the density ratio is low or blowing ratio is high. When the blowing ratios are 1.0 and 1.5, the improvements of averaged film cooling effectiveness are from 24.1% to 51.3% at the density ratio of 1, while the improvements are from 1.3% to 36.8% at the density ratio of 1.5. The reason is that the swirling in the coolant breaks the symmetricity of counter-rotating vortex pairs, leading to a deviation of streaks of film coverage and better attachment under high blowing ratios. However, due to the swirl, film cooling effectiveness for two rows of staggered holes is decreased, because the swirl turns the coolant flow direction and creates poor coolant coverage areas between adjacent holes, and weakens the coolant accumulation and re-attachment at further downstream area

Experimental study on thermal hydraulic characteristics of natural circulation loop under motion condition

• Comparatively studied thermal hydraulic characteristics of natural circulation loop under various motion conditions. • Obtained the periodic average frictional resistance coefficient and Nu. • Obtained the empirical relation of frictional resistance coefficient in turbulent region. • Under large flow rate conditions, rolling motion affects heat transfer characteristics mainly by affecting flow rate. • Some suggestions on the layout of marine natural circulation loop were given. In this present paper, an experimental research was carried out to investigate the influence of various motion conditions on the flow and heat transfer characteristics in natural circulation loops. A natural circulation loop was built on the six-degree-of-freedom motion experiment platform at Xi’an Jiaotong University (XJTU). First, it was found that the small acceleration heaving would not cause significant changes on flow rate of the natural circulation loop. Then, the influence mechanism of rolling motion on the natural circulation loop is different as the central axis of rolling is changed. The increase of additional inertial force along the coolant flow direction causes larger flow fluctuation. The periodic average frictional resistance coefficient under different working conditions is obtained, which is divided into flow fluctuation region ( Re < 5000) and turbulence region (5000 < Re < 35000). Last, according to the experimental data, a mathematical model to predict the periodic average frictional resistance coefficient which is suitable for the turbulent region is obtained, and the deviation between the predicted value and the experimental data is less than 20%. Beyond that, the period average Nu under different working conditions is also achieved. It shows that the rolling motion promotes the heat transfer with the small flow rate, while the rolling motion affects the heat transfer characteristics mainly by affecting the flow rate under the large flow rate conditions. Based on our research, some suggestions on the layout of marine natural circulation loop were given.

Study of the thermal behavior of a battery pack with a serpentine channel

To effectively enhance the thermal security of the Li-ion battery packs used in the electric vehicle industry, novel cooling systems equipped with serpentine channels are established. Then, the heat generation model is established and verified experimentally. In this research study, the structure of the cooling channel, the coolant velocity, the coolant temperature, and the coolant flow direction are considered to be the influencing factors. The results demonstrate that, by adopting the serpentine cooling channel, a better thermal conductivity can be obtained, and the type-B cooling system possesses a more reasonable structure. For different types of liquid cooling systems, the coolant temperature has a small influence on the temperature nephogram; however, for the same type of system, the coolant temperature strongly influences the temperature distribution. Similarly, the temperature difference is only related to the type of cooling system, with ∼6.09 and 5.53 K obtained for the type-A and type-B cooling systems, respectively. Furthermore, allowing the coolant in the serpentine cooling channels to flow in opposite directions can lower the value of the maximum temperature and temperature difference.

Liquid cooling vs hybrid cooling for fast charging lithium-ion batteries: A comparative numerical study

Lithium-ion batteries (LIBs) rely on efficient thermal management systems for their safe and reliable operation. Issues like overheating of LIBs due to excessive heat generation, thermal runaway and thermal propagation are still a challenge for engineers. To address the challenge, numerous cooling methods viz. liquid cooling, air cooling, cooling using phase change materials (PCMs), hybrid cooling (i.e., combining liquid and PCM cooling), etc., have been proposed by different authors in the past. Though all the above methods are best suited for LIBs under normal charging/discharging operations, their suitability for cooling fast charging LIBs remains unexplored. In general, the heat generation and risks due to thermal runaway in LIBs are relatively more during fast charging than normal charging. As regards hybrid cooling employing PCMs, the method is yet to be benchmarked, as PCMs themselves suffer from low thermal conductivity issues that may affect their overall thermal performance. The present work is an attempt to address the above-stated aspects. Detailed 3D numerical simulations on three different design configurations (D1-D3) of a prismatic LIB module consisting of four 10 Ah batteries fast charged at 8C rate are reported herein. Two different cooling methods viz. (i) liquid cooling employing a dielectric liquid coolant (STO-50) and (ii) hybrid cooling combining liquid dielectric and a PCM (RT35) are investigated, and their performances compared. The results reveal interesting facts on the ability of liquid cooling (D2) over hybrid cooling (D3) for fast charging LIBs. Parametric studies show that the coolant flow direction (horizontal/vertical) and thermal conductivity of the PCM (kpcm) have a profound influence on the cooling obtained compared to PCM’s melting temperature (Tm) and latent heat (λ). It is concluded that PCM based hybrid cooling is not suitable for cooling LIBs under fast charging unless kpcm is enhanced to 1 W/mK or above. A minimum coolant flow rate of 2 lpm is needed to limit the battery temperatures to 40 °C or below for the module under fast charging conditions. An increase in PCMs latent heat from 160 to 200 kJ/kg has no significant influence on the cooling achieved.

Pack-level modeling of a liquid cooling system for power batteries in electric vehicles

An efficient pack-level battery thermal management system is essential to ensure the safe driving experience of electric vehicles. In this work, we perform three-dimensional modeling of a liquid thermal management system for a real-world battery pack powering electrical vehicles. The effects of system structures, coolant flow direction layout, coolant flow rates, and inlet temperatures on the thermal performance are investigated. Under 2.0 C discharge, the system structure with interspersed cooling plates reduces the maximum temperature of the battery pack to 36.3 °C, which is 15% lower than that of the conventional design with bottom-mounted cooling plates. With the present system structure, around 62.2% of the heat produced by the batteries is dissipated. Moreover, the temperature difference is decreased by 10.5% with appropriately arranging coolant flow directions in different liquid cooling plates. In addition, increasing the coolant flow rate and lowering the inlet coolant temperature can greatly reduce the maximum battery temperature, while the latter parameter shows little influence on the pack temperature difference, which only changes 0.3 °C when the inlet temperature is decreased by 6 °C. This work provides valuable guidance for the development of pack-level liquid battery thermal management systems in electric vehicles.

Thermal-Mechanical Analysis of Beam Window and Beam Tube for ADS Granular Flow Target

A granular flow target coupled with a beam window was studied. The beam window isolates the accelerator from the target, making the system more secure and flexible. Preliminary analyses for the beam window and beam tube, including neutronics, thermal hydraulics, and structural mechanics were performed by Geant4 and ANSYS. The effects of geometry and coolant flow direction on the temperature field and the stress distribution of the beam window are studied. The results show that the maximum temperature can be reduced by 13% through optimization. Comparing the thermal deposition distribution of the beam tube with and without the beam window, we find that there is an extra peak due to the beam window. In addition, the effect of the cooling pattern on the temperature distribution of the beam tube is also studied. The results show that it is reasonable to arrange six U-shaped cooling channels. Detailed analyses show that the material temperature and the mechanical property of the beam window and beam tube meet the design standards, which confirm the possibility of granular flow target with a beam window for engineering application.

Cooling Characteristic of a Wall Jet for Suppressing Crossflow Effect under Conjugate Heat Transfer Condition

The leading edge is the critical portion for a gas turbine blade and is often insufficiently cooled due to the adverse effect of Crossflow in the cooling chamber. A novel internal cooling structure, wall jet cooling, can suppress Crossflow effect by changing the coolant flow direction. In this paper, the conjugate heat transfer and aerodynamic characteristics of blades with three different internal cooling structures, including impingement with a single row of jets, swirl cooling, and wall jet cooling, are investigated through RANS simulations. The results show that wall jet cooling combines the advantages of impingement cooling and swirl cooling, and has a 19–54% higher laterally-averaged overall cooling effectiveness than the conventional methods at different positions on the suction side. In the blade with wall jet cooling, the spent coolant at the leading edge is extracted away through the downstream channels so that the jet could accurately impinge the target surface without unnecessary mixing, and the high turbulence generated by the separation vortex enhances the heat transfer intensity. The Coriolis force induces the coolant air to adhere to the pressure side’s inner wall surface, preventing the jet from leaving the target surface. The parallel cooling channels eliminate the common Crossflow effect and make the flow distribution of the orifices more uniform. The trailing edge outlet reduces the entire cooling structure’s pressure to a low level, which means less penalty on power output and engine efficiency.

Effects of Coolant Flow Direction on Thermal Cooling Performance of Cylindrical Li-Ion Battery Pack with Water/Ferrofluid as Coolants

Effects of Coolant Flow Direction on Thermal Cooling Performance of Cylindrical Li-Ion Battery Pack with Water/Ferrofluid as Coolants