Cold Air Jet Research Articles

Head-neck local ventilation mode for long-narrow mine working face

To improve the thermal health state of workers in mines facing heat hazards and enhance cooling capacity utilization efficiency of mine ventilation, this study proposes a suitable air distribution for mine workers’ local cooling, taking into account the characteristics of long-narrow underground space and workers. The suggested air distribution involves harnessing underground cold air jets along with the mine’s crossflow (mainstream ventilation) to create a localized safeguard airflow around the worker’s head-neck, known as jet ventilation in crossflow (JVIC). The flow visualization experiment identified five flow patterns within a confined space. The study explores the impact of the velocity ratio (R) and confinement scale (C) on the evolution of JVIC flow patterns and presents a parametric description of the resulting flow field. Drawing on the microclimate control scope around mine workers’ head-neck, the study defines the effective cooling zone and the ineffective cooling zone as the boundaries for controlling JVIC air distribution within confined mine spaces. This clarifies the applicability of the air distribution form and introduces an evaluation model for assessing the cooling effect and the efficiency of cooling capacity utilization to manage the non-uniform underground environment.

Improving fluid flow and heat transfer of cryogen spray cooling using sweeping cold air jet

Industrial growth has led to increased electronic component integration, resulting in higher working temperatures and technological failures. Cryogen spray cooling (CSC) could reach cryogenic temperatures with fast-responding, but its heat transfer limitation due to liquid film deposition is potentially challenging. In this study, CSC cooling is incorporated with either a cold air jet (CAJ) or a sweeping jet (SWJ) to enhance heat and fluid flow. A K-type thermocouple is used for measuring the surface temperature, while the improved Duhamel theorem is used to estimate the heat flux. The spray and film intensities are recorded through the Mie-scattering methodology via high-speed camera. The results demonstrated that the CAJ/SWJ exhibited a significant reduction in surface temperature and film intensity, enhanced heat dissipation and spray efficiency compared to CSC cooling. More precisely, it demonstrated a 26%–36% heat transfer coefficient improvement, along with a 52% increase in the corresponding time of the critical heat flux. Meanwhile, the CAJ/SWJ intensifies heat transfer during film and transition boiling, inspired by the fast liquid film evaporations. In contrast, the SWJ is much preferred due to its wider and stable coolant coverage throughout film evaporation, as seen by its greater heat transfer coefficient. The combined results demonstrated the need to promptly eliminate the deposited film, as it holds the potential to serve as a pivotal resolution for the advancement of thermal management technology.

Intermittent cryogen spray cooling coupled with cold air jet for heat transfer enhancement and cryogen saving of laser dermatology

Laser dermatology has become a convenient technique for medical treatment. However, the epidermal temperature could be increased, leading to intolerable side effects. Cryogen spray cooling (CSC) is implemented to decrease epidermal damage, but the current CSC could increase cryogen wastage. In this study, the CSC is coupled with a cold air jet (CAJ) and is further subjected to intermittent strategy (ICSC) to mitigate film deposition, enhance heat transfer, and save cryogen consumption, through CAJ-to-spray interaction. The surface temperature is measured by a thin film type-T thermocouple, while the heat transfer is estimated by enhanced Duhamel theorem. The film intensity is captured by a digital single-lens reflex camera through the scattering-illumination technique. The results affirmed the benefits of using ICSC/ICSC-CAJ for laser dermatology. It demonstrated lower surface temperature, film intensity, and film resistance as well as higher convection and efficiency compared with CSC. ICSC-CAJ showed the lowest minimum temperature (1.8%–4.9% decrement) and film resistance (3.8%–17.5% decrement), the highest average convective coefficient (84%–92% increment) and efficiency (7%–9% increment) compared with ICSC. Lower frequency and moderate duties would be preferable for ICSC, while higher frequency and lower duties would be preferable for ICSC-CAJ. The efficiency increased with the saving (110% increment), which insights into our understanding of alternative enhancement toward cryogen saving strategy.

Transient behavior of liquid film deposition and surface heat transfer upon cryogen-spray cooling coupled with cold air jet

Cryogen spray cooling (CSC) has become a promising auxiliary-cooling technique for laser dermatology, which recently enhanced by cold air jet (CSC-CAJ). However, the CAJ-to-spray interactions could worsen the CSC performance and associated liquid film behavior. In this study, the transient behavior of liquid film deposition and surface heat transfer of CSC-CAJ was investigated. A high-speed camera was used to capture the film intensity through Mie-scattering technique. Fast-response TFTC thermocouple was used to measure the transient surface temperature. The large eddy simulation (LES) was used to extract the vortical structure, while the discrete-phase model (DPM) and Eulerian wall film (EWF) were coupled for the spray process and the transient film modeling. The results showed that the CAJ significantly enhanced CAJ-to-spray mixing and mitigated the film thickness (i.e., 32 % reduction). The CSC-CAJ had asymmetric film coverage due to induced counter-rotating vortex-pairs (CRVP) from the CAJ-to-spray interaction, which laterally clustered the spray. It permitted fast evaporation with dominant CAJ at the spray-periphery while permitting fast evaporation with dominant spray at the spray-core. Consequently, the CSC-CAJ showed 75 % temperature increment at the windward side and 15 % temperature reduction with 60 % heat transfer coefficient enhancement at the impingement point. The experimental film intensity and simulated film thickness were linearly correlated and significantly influenced the surface temperature. The combined results affirmed the significant impact of the CAJ on the transient film deposition, which potentially helps promote advanced cooling technology.

Supersonic jet noise and screech tone suppression using cross-wire

This experimental investigation is aimed at assessing how the introduction of a cross-wire at the exit of a CD nozzle influences the performance of a supersonic nozzle. The study focuses on cold air jets generated by De Laval nozzles equipped with cross-wires and baseline configurations, particularly at design Mach numbers of 1.5 and 1.75. The investigation involves collecting measurements from the noise field emitted by the cross-wire nozzle with a 2% obstruction at the exit. This passive control approach effectively reduces the occurrence of screech tones in both over-expanded and under-expanded conditions in the azimuthal plane at appropriate operating pressures. Various acoustic parameters, including sound pressure levels (SPL), Strouhal numbers, and the overall sound pressure level spectra (OASPL) are recorded. Schlieren imaging captures images of shock cell patterns, illustrating the impact of shock-associated noise. In comparison to a baseline nozzle, the results demonstrate that a CD nozzle equipped with a cross-wire proves to be a proficient screech tone suppressor, leading to an average reduction of up to 5 dB in OASPL in under-expanded and over-expanded scenarios.

Unstable spray pattern and cooling performance of cryogen spray coupled with cold air jet: An experimental study

This study focuses on the interactions between cryogen spray cooling (CSC) and cold air jet (CAJ), as well as the heat transfer performance upon CAJ-CSC impingement assisted for laser dermatology. CAJ is found to collapse downstream spray on the windward side, enhancing fluctuations of R1234yf spray width. Standard deviation (SD) distribution identifies that fluctuations at nozzle exit are attributed to ambient-to-spray interactions, which is also confirmed by the proper orthogonal decomposition (POD) analysis. The third POD mode highlights jet-to-spray interactions on the windward side of spray, yielding an enhanced instability by 1.72 times when CAJ flow rate (Q) increases from 4 to 8 m3/h. The closer location to the spray center on the same radial side corresponds to a faster thermal response and a lower temperature drop (ΔT). ΔT on the windward side decreases due to the considerable CAJ-induced droplet swarm drift. The heat transfer degrades on the leeward side due to the thermal gas boundary layer originating from the inclined CAJ sweep. Nevertheless, the CAJ with a small flow rate (4 m3/h) enhances the heat transfer at spray center, yielding a minimum surface temperature of −32.3 °C. Comparative investigation indicates R134a has superior cooling performance but more significant instabilities inside spray plume. R1234yf serves a relatively stable sub-cooling region within a radial distance of ±1 mm from spray center characterized by low spray fluctuations and high temperature drops at Q = 4 m3/h. Substitution of R1234yf for R134a still needs heat transfer enhancement for clinical considerations.

Investigation of unsteady flow behavior of cryogen-spray coupled with cold air jet

Cryogen spray cooling (CSC) coupled with cold air jet (CAJ) was used to mitigate Cryogen consumption. However, the CAJ interaction could affect the flow structure and spray's unsteadiness, permitting coolant intermittency. In this study, the unsteady behavior and flow structure of CSC coupled with CAJ have been qualified. During the experiment, CAJ at an inclined angle of 30°, temperature of 266 K, flowrate of 0.0, 3.0, 6.0 m3/h, and axial distance of 20 mm from the spray-nozzle was injected into a typical R-134a spray system. A high-speed camera was used to capture the unsteady spray through the Mie-scattering technique, while large-eddy simulation (LES) coupled with discrete-phase model (DPM) was used to predict the vortical structure. The spatial correlation was used to identify the coherent structures, while the proper orthogonal decomposition (POD) was used to demonstrate their unsteady signatures. The results showed a significant impact of the CAJ on the spray-pattern and associated unsteadiness, providing an asymmetric spray-plume at the CAJ windward side. The CAJ increased the fluctuating energies of the unsteady coherent structures by 32 % through CAJ-to-spray interactions. It contributed to the unsteady spray through the counter-rotating vortex-pairs (CRVP) at the lateral sides and hairpin vortices along the injection line. The CRVP clustered the spray in recirculating regions, introducing unsteady CAJ entrainment at the leeward and thermal field at the windward. This work could insight into our understanding of unsteady multiphase flow, promoting advanced cooling technology by uncovering their causes and effects.

Effect of zero penetration angle chevrons in supersonic jet noise and screech tone mitigation

Abstract An experimental study is conducted to determine the effect of chevrons with zero penetration angles at the CD nozzle exit on an emitted noise field. The implication of passive control is to reduce the blockage of the nozzle exit area with minimal engine thrust penalty. The cold air jets issued at design Mach numbers 1.5 and 1.75 from the De Laval nozzles of the circular section were investigated. This passive control eliminates screech tones at the over and ideally expanded conditions at 60° and 90° in the azimuth plane. The acoustic data measurements have also been observed for the chosen jet Mach numbers. The schlieren images reveal the shock cell pattern to eliminate the effect of shock-associated noise levels at supersonic jets. The results show that 10 chevrons with no penetration act as an effective eliminator of screech tone and noise suppression average ∆OASPL value up to 3 dB at Mach number 1.75.

Transient cooling performance of R134a flash-evaporation spray enhanced by cold air jet: A novel heat transfer enhancement method in laser dermatology

Nevus of Ota is a dermal hyperplastic pigmented disease with a morbidity rate of 0.1–0.6% in the Asian population. Laser surgery with the assistance of cryogen spray cooling (CSC) is a potential clinical treatment for nevus of Ota. However, for Asians with high melanin content, it is necessary to enhance cooling capability without the increment of refrigerant amount. In this study, a R134a flash-evaporation cryogen spray cooling coupled with cold air jet (CSC-CAJ) was developed to improve the cooling capacity. Epoxy resin was employed as a skin phantom to investigate the heat transfer characteristics of CSC-CAJ with different air jet parameters. Air jet parameters of CSC-CAJ are recommended to be Ta = -7 ℃, Va = 3 m3/h Za = 10 mm and Φa = 30°. Compared to CSC, CSC-CAJ could reduce the minimum surface temperature from -39 to -42.1 ℃, and the average value of convective heat transfer coefficient (h) of CSC-CAJ could increase from 8 to 12.9 W/(m2·k) or 61.3% for the recommended parameters. The liquid film residence time (τ) decreases from 301 to 234 ms or 23.3%. In addition, based on the visualization observation and surface heat transfer measurement, the frost formation phenomenon on the surface and the heat transfer enhancement mechanisms are well understood. Four modes, including Leidonfrost mode, no runoff mode, runoff mode and evaporation mode, are proposed to describe the four stages of CSC-CAJ. A non-dimensional correlation of normalized Nusselt number (Nu*) and time (t*) is summarized. The primary heat transfer enhancement mechanism of CSC-CAJ is found to be the increase in cryogen evaporation rate and the ratio of heat removal from the phantom surface to phase change latent heat.

Improving a rotating steel milling tool cooling by an air jet impingement flow

Turbulent jets impacting flat or curved surfaces have been the subject of several research reports in the literature over the past decades, and it is all about the improvement of localized heat and/or mass transfer in part of a system, to achieve this goal, it is necessary to understand the dynamic behavior of the fluid and its effect on local heat and/or mass transfer. The aim of our work is the numerical investigation of a steel milling tool cooling driven in rotation, emitting a heat flux caused by machining a surface, using an impacting cold air jet; Although the technology of cooling the machining tools exists by using liquid jets, it signaled that it is less economical and efficient then using the impinging air jet. The milling tool is cooled using forced convection produced by the air jet at different Reynolds numbers, this phenomenon is governed by the equations of conservation of mass, momentum, and energy. We adopted a mathematical model based on the finite volume method; the calculation code was validated using experimental results available in the literature. We obtained our results by varying the Reynolds number between 12,000 and 26,000 values for an impact distance ratio h/D = 4, and two configurations: one nozzle and two nozzles. Our results showed that when using two cold air jet inlets to cool the milling tool, the distribution of the Nusselt number on the surface is enhanced compared to the one nozzle case, mainly because of the two impact points of the jet on the surface. We also note that increasing the Reynolds number has a significant influence on the distribution of the averaged Nusselt number.

Effect of Film Position on the Flow and Heat Transfer Characteristics of Full-Ribbed Rotor Tip

This article numerically studied the flow and heat transfer characteristics of the full-ribbed fluted tip of a rotor with four types of distribution of cooling films, namely, the mid-arc uniform film at the bottom of the groove, the uniform film on full perimeter ribs, the uniform film on the suction-side rib, and the uniform film on the pressure-side rib. The evolution of secondary flow in the tip clearance under these four conditions was discussed, and the effect of cooling film position on the tip leakage flow and tip heat transfer was also studied. The results showed that films in the ribs were more conducive to lower leakage flow rates and can reduce total energy losses. Adding cooling films on the single-side rib had a better effect on the control of leakage flow rate, with the most significant improvement due to adding cooling films on the suction-side rib. The mid-arc jet at the bottom of the groove can greatly contain the swirl structure in the groove, but it weakened the air sealing effect, which led to a rise in leak flow. The main reason for the swirl structure in the groove was that the leakage flow from the tip of the rotor was blocked by the cold air jet from the cooling films to form a backflow. The vortex inside the groove was the most violent when the air film hole was set on the pressure side, so the loss coefficient was higher, while the vortex inside the groove was the simplest when the cooling film hole was set on the suction side, and the energy loss caused by the vortex inside the groove was the least, and this structure was more favorable to the cooling of the leaf tip. It was also found that the uniformity of the outlet airflow angle distribution improved in the presence of cold air injection on the tip of the rotor.

Effect of Cold Air Jet Cooling on Weld Quality in Friction Stir Welding of AA2024-T351

Effect of Cold Air Jet Cooling on Weld Quality in Friction Stir Welding of AA2024-T351

Transient Cooling Performance of R134a Flash-Evaporation Spray Enhanced by Cold Air Jet: A Novel Heat Transfer Enhancement Method in Laser Dermatology

Transient Cooling Performance of R134a Flash-Evaporation Spray Enhanced by Cold Air Jet: A Novel Heat Transfer Enhancement Method in Laser Dermatology

Influence of the Multiple Layers Application and the Heating of Silane on the Bond Strength between Lithium Disilicate Ceramics and Resinous Cement.

Objective This study aimed to evaluate the bond strength between lithium disilicate ceramic and resinous cement when silane (Prosil, FGM) was applied in different amounts of layers under heating or not. Materials and Methods Sixty IPS E-max CAD ceramic (Ivoclar) was used. They were conditioned with 10% hydrofluoric acid for 20 seconds. The samples were distributed in six groups ( n = 10): 1Sil, 1 layer of silane without heating; 1SilAq, 1 layer of silane with heating; 2Sil, 2 layers without heating; 2SilAq, 2 layers with heating; 3Sil, 3 layers without heating; and 3SilAq, 3 layers with heating. After each layer, a jet of cold air was applied for 20 seconds in groups 1Sil, 2Sil, 3Sil, and jet of hot air (50°C) in groups 1SilAq, 2SilAq, and 3SilAq. Subsequently, an adhesive layer was applied, and fourcylinders were made on the ceramic with a resin cement AllCemVeneer and photoactivated for 20 seconds. The samples were stored at 37°C for 24 hours and analyzed to the microshear test at EMIC. Statistical Analysis Data were submitted to ANOVA and Tukey’s test (α = 0.05). Results The results showed that there was no statistical interaction between the factors studied. The “heating” factor was not statistically significant; however, the “silane layers” factor showed differences between groups. The analysis of the results showed that the use of one (66%) or two layers (67%) of silane regardless of heating, produced higher values of bond strength, when compared with the group of three layers (62%). Conclusion The use of silane with one or two layers provided a greater bond strength between lithium disilicate ceramic and resinous cement and that the heating did not influence the results.

Effect of porous partition height on thermal performance of a ventilated cavity using LBMMRT

The objective of this work is to study the effect of the thickness of a porous separation on the thermal performance in a cavity with displacement ventilation. The cold air jet enters and exits through two openings located in the lower and upper parts of the left wall and the right wall respectively. The other horizontal walls are also adiabatic. The hydrodynamic and thermal characteristics of the transfer were studied for three configurations with the same aspect ratio L/H=2. The height Hp of the porous separation was varied between 0.2 and 0.8 where is placed in the center of the cavity. The transfer rates on the active wall for the thicknesses were studied for different permeability therefore different Darcy numbers varying over an interval:10-6≤Da≤10. The dimensionless Rayleigh and Reynolds numbers were taken from the rows: 10≤Ra≤106 and 50≤Re≤500. The governing equations of momentum and energy were solved by the Lettice Boltzmann Multiple Relaxation Time Method (LB-MRT) D2Q9 for the velocity field and D2Q5 for the temperature field. In order to take into account the introduction of the porous medium, an additional term is added to the standard LB equations based on the generalized model (Darcy model extended to Brinkman-Forchheimer).

Impinging jet into an open trapezoidal cavity partially filled with a porous layer

A confined slot jet impinging into an open trapezoidal cavity containing a porous layer is investigated. To make the study accounts for a practical problem, constant heat flux is applied at the bottom of the cavity which is covered by a thin porous layer subjected to a cold impingement air jet. Darcy-Forchheimer model is used for the porous layer and its porosity range was taken to ensure thermal equilibrium between the fluid and the porous matrix. Finite element method is adopted for the numerical solution. Results show that the best thickness of the porous layer should be as minimal as possible, where increasing the porous layer thickness from 0.1 to 0.3 reduces the average Nusselt number by 17% when Darcy number is 10−3. It is found also that the trapezoidal cavity serves in attaining the fully developed condition early.

CALCULATION OF THE FREQUENCY CHARACTERISTICS AT THE POINT OF THE ACOUSTIC FIELD OF A FREE SUPERSONIC JET OF A ROCKET ENGINE

The paper studies the frequency characteristics of the acoustic field of a free supersonic jet of a powder rocket engine. The results were compared with similar results of free supersonic cold air jets by a number of other authors.

ON APPLICABILITY OF ADDITIVE TECHNOLOGY IN PRODUCING NOZZLES FOR JET NOISE INVESTIGATIONS

The split-type conical nozzles with replaceable exit sections with diameters of 30, 40 and 50 mm were designed and produced from steel by machine turning. In addition, the replaceable output parts of the nozzles with the same diameters were produced by additive technology (fused deposition manufacturing) from ABS plastic. In the acoustic anechoic chamber, the noise measurements of a single-stream cold air jet for all the nozzles at jet velocities in the range of 0.3-0.7 Mach numbers were carried out. The noise measurements were performed on distance of 2 m from the center of the nozzle exit section at angles from 30 to 105o. For different directions of noise radiation and different velocities of the jet, the power spectral density and overall sound pressure level were determined. The obtained results demonstrates that the jet noise for nozzles with diameter of 40 and 50 mm from steel and ABS plastic differs by no more than 1 dB, which is within the measurement error for these types of experiments. The nozzles with diameter 30 mm have a higher difference in noise, which can be explained by the more sensitivity of a nozzle with a small diameter to the deviations of geometric parameters when it produced by additive technology.

Plasticity Behavior of the Steel Depending on the Cooling Regimes in the Case of a Non-Conventional Treatments

Plasticity of the steel -as an important property of the material -has the role to show the capability of the steel to support plastic deformation necessary to obtain pieces for industry. Plasticity can be influenced by cooling regimes from higher temperatures in the cases of non-conventional treatments. An important factor is the cooling speed of the steel from higher temperatures during non-conventional treatment applied after lamination of the material. To modify the speed of the cooling, the medium of the cooling has been changed. The samples of the steel have been cooled in normal condition, in metallic box and using a jet of cold air. The evolution of the plasticity was studied taking in consideration the evolution of some characteristics. This paper might be considered as a review of the researches from the last years.

Cooling Effects of Cold Mist Jet with Transient Heat Transfer on High-Speed Cutting of Titanium Alloy

High cutting temperature will lead to a short tool life and a poor machining quality during titanium alloy machining. High efficiency cooling methods are always required to reduce the cutting temperature. In this paper, heat transfer experimental platform was set up and one-dimension heat transfer model was established to get critical heat flux (CHF) used to evaluate the cooling capacity of the cooling methods. The effect of cold mist jet (CMJ) was evaluated by transient heat transfer tests and titanium alloys cutting experiments, comparing with cold air jet, flood cooling and minimum quantity lubricant. The experimental results revealed that CMJ with a high CHF has a stronger cooling capacity than other cooling methods and has greatly reduced the cutting temperature in high-speed cutting of titanium alloys. And tool life and surface quality have been both improved with CMJ. This paper offered a way for evaluating the cooling effects of different cooling methods with heat transfer experiments.