Flow Boiling Heat Transfer Enhancement Research Articles

An improved wall boiling model for numerical simulation of subcooled flow boiling on a new hybrid micro/nanostructured surface

Previous studies have demonstrated that micro/nanostructured surfaces have great potential for heat transfer enhancement. However, simulating subcooled flow boiling on such surfaces is difficult owing to the lack of proper bubble characteristic parameter models, because most models used in flow boiling simulations were developed based on smooth surface conditions, which may limit their applications in engineering design. In this study, we improved upon one validated bubble characteristic parameter model suitable for subcooled flow boiling on smooth surfaces to adapt to the new hybrid micro/nanostructured surfaces proposed by Huang et al. [“Experimental investigation of a new hybrid structured surface for subcooled flow boiling heat transfer enhancement,” Appl. Therm. Eng. 192, 116929 (2021)]. The new bubble characteristic parameter model incorporates both basic correction terms to account for boiling bubble behaviors and ad hoc parameters to account for other unknown effects. Through sensitivity analysis and detailed calibration, the model was simplified to a set of correlations and only one constant parameter. With this improved model, subcooled flow boiling heat transfer simulations were conducted for three target surface specifications under conditions of 4–10 MW/m2 incident heat flux and 1–5 m/s flow velocity and the related heat-transfer mechanisms were further compared and discussed. The maximum error between the simulation and experimental results remains less than 3.5%, indicating that the established model has considerable accuracy in predicting the heat transfer performance for this type of micro/nanostructured surface in high-heat-flux engineering design applications. The heat transfer enhancement for subcooled flow boiling on this type of hybrid micro/nanostructured surface is greatly beneficial owing to its proper organization of convection, evaporation, and quenching heat transfer.

Microfluidic controllable production and morphology-independent phase-change heat transfer of boehmite nanofluids

Enhanced boiling heat transfer through nanofluids represents a significant approach to improving the cooling performance of electronic devices. However, the preparation of nanofluids poses challenges regarding production efficiency, morphology control, and structural stability. In addition, the underlying mechanism of flow boiling heat transfer enhancement of nanofluids, particularly changes in vapor behavior, remains a limited understanding. This study presents a facile and straightforward microfluidic strategy for the controllable synthesis of nanofluids toward efficient heat transfer enhancement. The microfluidic flow synthesis approach effectively enhances mixing efficiency at various flow rates, enabling continuous and high-throughput synthesis of boehmite nanofluids with different morphologies. The synthesized nanofluids exhibit long-term stability (>50 days) at room temperature. Both rod- and sphere-shaped nanofluids under equivalent concentrations can significantly improve critical heat flux (CHF) and maximum heat transfer coefficient (HTC) across various operating parameters, but with similar enhancement characteristics. Analysis of the period of bubble growth and burst, bubble size distributions, number of bubbles, and vapor phase fractions reveals that rod- and sphere-shaped nanofluids could shorten the period, enhance surface wetting, increase nucleation sites, and reduce bubble size efficiently and similarly. Furthermore, the effect of the concentration of nanofluid on phase-change heat transfer performance is systematically explored. 0.01 wt% nanofluid behaves the most remarkable enhancement, CHF and maximum HTC are improved by 24 % and 14.4 %, respectively. These findings not only shed new light on the controllable synthesis of functional nanomaterials but also provide important guidelines for the rational design of nanofluids for efficient two-phase cooling of electronic devices.

A Review on Flow Boiling Enhancement on Textured Surfaces

It is widely established that flow boiling, being a direct cooling technique also employing the latent heat of the fluid, has the potential to be more efficient than being useful in single-phase conventional cooling methods. This results in considerable potential for thermal management in many fields like microelectronics, space technology, thermal power plants, etc. The increasing demand for heat dissipation, consequent to component miniaturization, has pushed the development of new strategies for enhancing heat transfer efficiency, such as employment of functionalized surfaces. This review aims to describe in detail the current status of technology related to flow boiling heat transfer enhancement via micro/nanoscale surface functionalization. Key objectives are an increased nucleation site density and enhanced bubble dynamics. The vast majority of findings show favorable heat transfer performance, evidenced by an earlier onset of boiling (ONB), an improved flow boiling heat transfer coefficient (HTC), and an ameliorated critical heat flux (CHF). Increased pressure drop is a serious concern in certain application cases. Nanoscale textures mainly enhance capillary wicking to nucleation sites, thus being more effective in combination with microscale textures that define fixed nucleation sites. Degradation effects need to be more thoroughly and systematically characterized for application cases. Extra effects related to the manufacturing process can be easily overlooked, but one should be aware of their possible existence when drawing conclusions. Finally, the implementation of enhanced surfaces in mainstream applications is hindered by the absence of general predictive design tools for different channel configurations/materials, fluids, and operating conditions. A more universal understanding of the basic mechanisms involving texture geometry is needed in this aspect.

Flow boiling heat transfer enhancement in vertical minichannel heat sink with non-uniform microcavity arrays under electric field

Effects of electrohydrodynamics (EHD) on the bubble behavior and enhanced heat dissipation mechanism for four surface configurations and various heat fluxes in an upward flowing minichannel heat sink were studied experimentally. Three different minichannel configurations with microcavities array were fabricated using direct metal laser sintering. Flow boiling experiments of nanofluid (SiO2-R141b) under different nonuniform electric fields were performed. Homogeneity test of Variance Analysis was introduced to analyze the effect of design and operation parameters on heat transfer. Coupled effect of microcavity and electric field was further found to improve boiling heat transfer efficiency. Visualization shows that the diameter of small bubbles leaving microcavity increased under electric field. The bubbles wiggled between wire electrode and heated wall by action of electric field in bubbly flow district. Furthermore, the mechanism of perturbation of small bubbles flowing along the heating wall under electric field was discussed. It was revealed that the micro-concave cavity array distributed dense downstream and sparse upstream had a large impact on heat transfer coefficient (HTC), especially under electric field. The maximum synthetic enhancement factor of 2.11 was obtained for given flow conditions. Furthermore, a new predictive equation was proposed by a nonlinear regression equation to consider the effects of micro-concave cavity distribution and electric field factors on thermal transfer performance, and this correlation can predict the HTC affected by non-uniform micro-concave arrays well within the ± 10% error band.

Flow boiling heat transfer enhancement via micro-pin-fins/ZnO nanorods hierarchical surface

In this study, ZnO nanorods were synthesized on micro-pin-fins by a simple and inexpensive aqueous solution method for flow boiling heat transfer enhancement. The morphology, chemical composition and wickability of the micro/nano hierarchical surface (PF+NR) were characterized. The heat transfer performance of PF+NR was investigated under various flow velocities together with a micro-pin-finned surface (PF), a ZnO nanorod-coated surface (NR) and a smooth surface (S). The results indicated that the boiling performance is improved significantly by the hierarchical surface and demonstrated a synergism between the microstructure and the nanostructure. The maximum boiling heat transfer coefficient of PF+NR is increased by 63%, 76%, and 158% compared with PF, NR and S, respectively. The critical heat flux is also increased by 31%, 33%, and 74%, respectively. The onset of nucleate boiling temperature of PF+NR is decreased compared with that of the other surfaces. The comprehensive improvement of boiling heat transfer performance is attributed to the cooperation of structures in different scales: the nanorods provide abundant nucleation sites, and the micro-pin-fins improve the liquid replenishment capacity and suppress the lateral coalescence of large bubbles at high heat fluxes.

Experimental investigation of surface finishing technique impact on subcooled flow boiling heat transfer enhancement: sandpapering and sandblasting

Experimental investigation of surface finishing technique impact on subcooled flow boiling heat transfer enhancement: sandpapering and sandblasting

Flow boiling characteristics in plain and porous coated microchannel heat sinks

Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated with HFE-7200. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 – 400 kg/m2 s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 206.6 kW/m2. Flow visualisation results and SEM surface analysis are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide. The plain channels had fewer cavities for a given area and these were larger, i.e. up to 6 µm. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern development occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux, with earlier flow pattern transitions in the coated heat sink. Enhancement in microchannel flow boiling heat transfer was shown to be influenced by mass velocity and may reach up to 44% at low heat fluxes, where the nucleate boiling mechanism is dominant. It diminishes with increasing heat flux, corresponding to nucleate boiling suppression following flow regime transition. Pressure drop increase posed by the coated heat sink is relatively small in terms of overall system power consumption but appears to be influenced by mass flux, pressure oscillations as well as channel rewetting behaviour.

Flow boiling heat transfer enhancement under ultrasound field in minichannel heat sinks

The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer.

Flow Boiling Heat Transfer Enhancement Under Ultrasound Field in Minichannel Heat Sinks

The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer.

Numerical study of flow reversal during bubble growth and confinement of flow boiling in microchannels

The flow boiling instability caused by confined bubbles generated during the flow boiling process in parallel microchannels leads to flow reversal and earlier critical heat flux (CHF), which is a major problem with the extensive application of two-phase flow in microchannel heat sinks. In this study, single bubble growth in one of the two nearby microchannels is investigated through numerical methods. The VOF method, Hardt's phase-change model, conjugate heat transfer between solid and fluid domains were adopted within a self-developed OpenFOAM solver. After the growing bubble being confined by the sidewalls, the flow path of the channel is blocked thus the inlet liquid flow tends to flow into the other channels instead of pushing the confined bubble, which caused the confined bubble to extend towards upstream and flow reversal. By increasing the mass flux, the flow reversal is suppressed, but the flow boiling heat transfer enhancement is minimized due to the less evaporation area. The method of combination of microchannel and microgap (CMC) has little effect on flow reversal until the bubble enters the microgap. And its thermal performance is worse than the original heat sink. The confined bubble in the finned microchannel (FMC) heat sink would flow into the nearby channel through the secondary channel between the fins thus prevent flow reversal. Furthermore, the evaporation area between the bubble and solid walls increases, thus significantly lower thermal resistance (for about 40%) can be obtained. Considering the overall flow stability and heat transfer performance, the finned microchannel is recommended to be fabricated in industrial applications.

Convective Flow Boiling Heat Transfer Enhancement with Aqueous Al2O3 and TiO2 Nanofluids: Experimental Investigation

Convective Flow Boiling Heat Transfer Enhancement with Aqueous Al2O3 and TiO2 Nanofluids: Experimental Investigation

Experimental investigation of a new hybrid structured surface for subcooled flow boiling heat transfer enhancement

A new scheme with hybrid submillimetre/micro/nano-structured surfaces providing both high subcooled flow boiling performance and relatively low flow resistance was proposed to develop highly efficient and reliable water cooling schemes for plasma facing components (PFCs) in future nuclear fusion engineering. Both comparative microscopic surface structure pattern observations and macroscopic mockup subcooled flow boiling experiments under fusion reactor like heat flux conditions were conducted. The proposed technique can create hybrid structures favourable for subcooled flow boiling enhancement: including increasing bubble generation sites and multiscale instability etc. We demonstrated that the heat transfer coefficients of manipulated surfaces in the subcooled flow boiling state were almost doubled and that the critical heat flux (CHF) values were promoted by 80–200% under the tested flow conditions compared with under smooth surface conditions, with the flow path pressure drop deteriorating by less than 12%, indicating better capacities of high heat flux removal and low flow path resistance than those of existing PFC cooling schemes. The results encourage further optimization and development of this new scheme for practical application in nuclear fusion engineering.

Experimental investigation of flow boiling heat transfer enhancement under ultrasound fields in a minichannel heat sink

Ultrasound is considered to be an effective active heat transfer enhancement method, which is widely used in various fields. But there is no clear understanding of flow boiling heat transfer characteristics in micro/mini-channels under ultrasonic field since the studies related are limited up to now. In this paper, a novel minichannel heat exchanger with two ultrasonic transducers inside the inlet and outlet plenum respectively is designed to experimentally investigate the impacts of ultrasound on flow boiling heat transfer enhancement in a minichannel heat sink. Flow visualization analyses reveal that ultrasound can promote rapid bubble motion, bubble detachment from heating wall surface and thereby new bubble generation, and decrease the length of confined bubble. Furthermore, the flow boiling experiments are initiated employing working fluid R141b at different ultrasonic parameters (e.g., frequency, power, angle of radiation) and heat flux under three types of ultrasound excitations: no ultrasound (NU), single inlet ultrasound (IU), inlet and outlet ultrasound (IOU). The results indicate that ultrasound has obvious augmentation effects on flow boiling heat transfer even though the intensification effects will be limited with the heat flux increases. The higher ultrasonic power, the lower ultrasonic frequency and the higher ultrasonic radiation angle, the better intensification efficiency. The maximum enhancement ratio of have in the saturated boiling section reaches 1.88 at 50 W, 23 kHz and 45° under the experimental conditions. This study will be beneficial for future applications of ultrasound on flow boiling heat transfer in micro/mini-channels.

Effects of pin and wire electrodes on flow boiling heat transfer enhancement in a vertical minichannel heat sink

In this paper, electrohydrodynamic (EHD) enhancement of flow boiling heat transfer in a vertical minichannel heat sink was investigated experimentally. The pin and wire electrodes are used to create different kinds of non-uniform electric fields. Flow boiling experiments of refrigerant R-141b under electric fields are conducted under various applied voltages. The results show that the application of pin and wire electrodes can promote the Onset of Nucleate Boiling (ONB) and effectively enhance heat transfer coefficient. For the pin and wire electrodes, the EHD enhancement effect increases with the increase of applied voltage. At the same heat flux, heat transfer enhancement for pin electrode is higher than that for wire electrode at low applied voltage (U ≤ 400 V), but is lower than that for wire electrode at high applied voltage (U ≥ 550 V). Moreover, the obtained maximum enhancement ratio for pin and wire electrodes could reach 1.72 and 1.80, respectively. The influence area of pin electrode is limited, whereas the influence area of wire electrode is the entire electrode length at U ≥ 400 V. Based on the simulation and experiment results, the enhancing mechanism of EHD on flow boiling heat transfer in minichannel was discussed.

An experimental investigation on flow boiling heat transfer enhancement using Cu-TiO2 nanocomposite coating on copper substrate

Flow boiling heat transfer performances of Cu-TiO2 nanocomposite coated copper surfaces have been studied experimentally in this work for its potential use in heat transfer applications. Experimental studies are performed in a bottom surface heated minichannel with DI water as the coolant. Thin Cu-TiO2 nanocomposite coated copper surfaces are developed using electrocodeposition technique. Coated surfaces developed through this technique have varied surface properties such as wettability, porosity, crystallinity, etc. The developed coated surfaces are characterized with respect to porosity, mean pore diameter, roughness, contact angle, and coating thickness. Flow boiling heat transfer experiments are performed at different mass fluxes in an experimental setup developed for this purpose. The results obtained with bare copper surface are used as the reference data for comparing the performances of the coated surfaces. The Cu-TiO2 coated copper surfaces are found to augment single-phase heat transfer coefficient slightly, whereas the enhancement in the two-phase region is up to 94% depending on the mass flow rate of coolant and heater surface temperature. The critical heat flux (CHF) also augments for the nanocomposite coated surfaces up to 92%. The augmentation in CHF and heat transfer coefficient of Cu-TiO2 coated surfaces is due to the enhanced surface roughness, wettability improvement, and presence of high-density active nucleate site on the surface. In order to have appreciation of the results obtained in present work, the boiling curves and CHF against mass flux for the bare and coated surfaces are compared with previously published experimental results. Thus, the arrangement of the minichannel and porous surface offers a capable choice for compact small size cooling devices due to decreased wall superheat temperature, high CHF, and high heat transfer coefficient.

Experimental comparison and visualization of in-tube continuous and pulsating flow boiling

This experimental study investigated the application of fluid flow pulsations for in-tube flow boiling heat transfer enhancement in an 8 mm smooth round tube made of copper. The fluid flow pulsations were introduced by a flow modulating expansion device and were compared with continuous flow generated by a stepper-motor expansion valve in terms of the time-averaged heat transfer coefficient. The cycle time ranged from 1 s to 7 s for the pulsations, the time-averaged refrigerant mass flux ranged from 50 kg m−2 s−1 to 194 kg m−2 s−1 and the time-averaged heat flux ranged from 1.1 kW m−2 to 30.6 kW m−2. The time-averaged heat transfer coefficients were reduced from transient measurements immediately downstream of the expansion valves with 2 K and 20 K subcooling upstream, resulting in inlet vapor qualities at 0.05 and 0.18, respectively, and covered the saturated flow boiling range up to the dry-out inception. Averaged results of the considered range of vapor qualities, refrigerant mass flux and heat flux showed that the pulsations at low cycle time (1 s) improved the time-averaged heat transfer coefficients by 5.6% and 2.2% for the low and high subcooling, respectively. However, the pulsations at high cycle time (7 s) reduced the time-averaged heat transfer coefficients by 1.8% and 2.3% for the low and high subcooling, respectively, due to significant dry-out when the flow-modulating expansion valve was closed. Furthermore, the flow pulsations were visualized by high-speed camera to assist in understanding the time-periodic flow regimes and the effect they had on the heat transfer performance.

Effects of electric field on flow boiling heat transfer in a vertical minichannel heat sink

Effects of applied DC electric field on flow boiling heat transfer in a vertical minichannel heat sink for two mass velocities and different heat fluxes are investigated experimentally. R141b is employed as working fluid. The heat sink contains eight rectangular minichannels, as well as both width and height of a single minichannel are 2 mm. Wire electrode is used and placed in the center of minichannel to create a non-uniform electric field. Flow visualization is performed using a high-speed video camera to analyze the interfacial features with and without electric field. Furthermore, the mechanism of two-phase flow boiling heat transfer enhancement under an electric field is explored. The results show that the onset of nucleation boiling slightly shifts towards downstream under electric field conditions. Electric field brings more vapor production, leading to early transition from bubbly flow to slug flow and early transition from slug flow to churn or annular flow. Electric field causes the bubbles to oscillate between the wall and the wire electrode, or renders the bubbles intermittently to touch the heated wall in the bubbly flow region. In the slug flow region, the surface of the elongated bubble intermittently touches the heated wall, and the interfacial shape irregularly changes with the bubble growing under electric field conditions. Overall, applying electric field can enhance the flow boiling heat transfer performance in the minichannel heat sink, and the enhancement effect is more significant in the bubbly and slug flow region. Moreover, the maximum enhancement ratio of 2.48 is obtained under present experimental conditions.

Facile Fabrication of Nanostructured Microchannels for Flow Boiling Heat Transfer Enhancement

ABSTRACTFlow boiling in microchannels promises high heat transfer due to the combined effect of latent heat of vaporization and forced convection in confined spaces. However, flow boiling based miniaturized thermal management devices are limited due to instability induced dryout. While several efforts have been made to delay instabilities via advanced surface modification techniques, there is a need to expand the scope of applications by developing low-cost and scalable fabrication technologies for commonly used heat exchanger materials. In this paper, we use a facile and self-limiting chemical oxidation technique for fabricating sharp needle-like superhydrophilic CuO nanostructures within six parallel 500 × 250 µm2 microchannels spread uniformly over a 1 × 1 cm2 area in a copper heat sink. We demonstrate heat transfer enhancement with nanostructured microchannels (NSM) without any appreciable change either in the average pressure drop or the fluctuations in comparison to baseline plain wall microchannels (PWM). Analysis of the high-speed images was performed to attribute the enhancement with NSM to the presence of a capillarity-fed thin-film evaporation regime, which otherwise was absent in PWM. We believe that these results are encouraging and suggest that the heat sink geometry can be optimized to investigate the true potential of nanostructured microchannels.

Experimental study on the characteristics of CHF and pressure fluctuations of surfactant solution flow boiling

The critical heat flux (CHF) and presure fluctuation characteristics of surfactant solution flow boiling were studied experimentally and compared to that of pure water. The tested fluid was an aqueous solution of cetyltrimethyl ammonium chloride (CTAC) with addition of sodium salicylate (NaSal) at the same mass concentration. Tests were performed in a flow channel with 6.0×3.5mm cross-section over flow rate range of 192–406ml/min, at 80°C inlet temperature and 101.3kPa outlet pressure. The present experimental results show that, the CTAC/NaSal solutions behave an increased CHF: the value of CHF kept nearly constant at a level about 1.26 times of that of pure water when the solution concentration was beyond 100ppm (ppm: part per million). And the CTAC/NaSal solution had stronger pressure fluctuations than water. Visualization of the flow boiling process shows that bubbles in CTAC/NaSal solution have smaller diameters and larger population density than that in water, which may be the reason of higher CHF for surfactant solution case. And the bubbles in CTAC/NaSal solution existed for a longer time than in water, which was about ten times of the bubbles in water. These bubbles increased flow velocity and enhanced disturbances, which may be another reason of flow boiling heat transfer enhancement and stronger pressure fluctuations.

Flow boiling characteristics of micro-grooved channels with reentrant cavity array at different operational conditions

Micro-grooved channels with reentrant cavity array (MGRA) was developed using the orthogonal Ploughing/Extrusion method (P/E) for the purpose of flow boiling heat transfer enhancement. The flow boiling tests were conducted using deionized water as working fluid with variation in the heat flux, mass flux (G=170, 230 and 290kg·m−2·s−1) and inlet liquid subcooling (△Tsub=10, 25 and 40°C). The heat transfer performance for the MGRA was compared with that for the micro-grooved channels (MG) without reentrant cavities on the side wall. The results indicated that the MGRA yielded higher heat transfer coefficient (HTC) than the MG due to the enhanced nucleation and interruption of the boundary layer. Despite the higher pressure drop, the MGRA took the advantage of mitigating the two-phase instability thanks to the sharp corner of the cavities and capillary-driven liquid intake. Increasing the mass flux increased the HTC while there existed an optimum inlet liquid subcooling for HTC maxima. The pressure drop increased with increase of mass flux while a reverse trend was observed for the inlet temperature.