Smooth Microchannel Research Articles

Experimental investigations of flow boiling heat transfer performance in finned micro-channels

The flow boiling heat transfer and pressure drop in rectangular-finned micro-channels (i.e. micro-channel with rectangular fins) were experimentally investigated at five different fin-heights. At three different mass fluxes and a range of heat fluxes, heat transfer coefficients and pressure drops in the micro-channels were measured. The flow patterns during the flow boiling process were visualized to estimate the flow regime transitions in the micro-channels. The experimental results showed that the micro-channel with 500 μm height fins (i.e. tip clearance = 100 μm) performs the highest heat transfer and shows the highest resistance to flow boiling instability among the studied cases. Compared to the smooth micro-channel, the averaged heat transfer coefficient in the micro-channel with 500 μm height fins has been improved by up to 133.9 % at the same wall temperature. Compared to the micro-channel with 600 μm height fins (i.e. tip clearance = 0 μm), the averaged heat transfer coefficient in the micro-channel with 500 μm height fins has been improved by up to 45.8 %. At the initial point of flow instability, the micro-channel with 500 μm height fins has the highest pressure drop fluctuation frequency, the highest surface wetting frequency, and the lowest pressure drop variation coefficient among the studied cases. However, the micro-channel with 600 μm height fins has the lowest pressure drop fluctuation frequency and highest pressure drop variation coefficient, hence experiencing the longest dry-out process among the studied cases.

Flow analysis and fabrication of micro scale controlled surfaces by ultrashort pulse laser for microfluidic device applications

Microfluidics finds a wide range of industrial applications mainly in fields such as biomedical, clinical, electrical, and thermal engineering. Microchannels are the building blocks of most microfluidic devices. Thus, it becomes evident to understand the properties and the effect of the surfaces on the outcomes of devices. In this study, the effect of the shape of microchannels and the roughness elements on the flow is evaluated. Microchannels with three cross-sections and roughness elements of three different geometries are considered. The effect of varying Reynolds numbers on the flow parameters such as Friction factor, Nusselt number, Poiseuille number, pressure drop and surface temperature at the base of the substrate is studied. It is observed that rough microchannels and rectangular roughness elements showed a higher Nusselt number than smooth microchannels. The rough microchannels with triangular roughness elements showed higher friction factors. The surface temperature is higher for the smooth microchannels with triangular roughness elements. Furthermore, to demonstrate the manufacturability of microfluidic channels with controlled surfaces and to validate the preliminary results of the numerical simulations, the ultrashort pulse laser micromachining technique is used to fabricate the microchannels with structures on Polymethyl Methacrylate (PMMA) material.

Heat transfer enhancement and flow resistance reduction characteristics of non-uniform electrohydrodynamic conduction pumping in microchannels

In this study, the thermo-hydraulic performance of non-uniform electrohydrodynamic (EHD) conduction pumping in rectangular and straight microchannels was experimentally investigated. Electrodes are arranged in two ways: dimensions constant, spacing varies (Cases 1–3); both dimensions and spacing vary (Cases 4–6). The results show that EHD conduction pumping significantly enhances heat transfer performance, increasing the average Nusselt number (Nu) by 17.9 %-87 % compared to a smooth microchannel. As Re increases, the inertial force of the fluid dominates, diminishing EHD-induced vortices and limiting heat transfer enhancement. EHD conduction pumping enhances heat transfer by creating vortices that mimic solid pseudo-roughness microelements, increasing fluid disturbance and mixing near the wall. However, these vortices also impede the flow, resulting in a trade-off with increased pressure drop resistance. This paper innovatively adopts an arrangement of increasing electrode size (non-uniform), which effectively reduces the pressure drop resistance in the microchannel while maintaining the advantage of enhanced heat transfer. Cases 4, 5, and 6 exhibit significantly higher comprehensive heat transfer performance (η) than Case 0 (without electrodes), especially at low Re. While the inertial force at high Re suppresses vortex formation and heat transfer enhancement, η remains superior to Case 0. Notably, Case 5 maintains a minimum η of 1.18 even at Re = 1000. Furthermore, this paper proposes an equivalent slip drag reduction mechanism to elucidate the drag reduction effect of the increasing electrode size arrangement. Heat transfer augmentation and pressure drop reduction, as mentioned here in microchannels, are crucial for electronics cooling and microfluidic devices.

A comparative study of flow boiling performance in smooth and multi-stage enhanced open microchannels

Flow boiling in open microchannels is currently one of the research focuses in the field of phase-change heat transfer. The present paper described a comparative and experimental research on flow boiling heat transfer of SF-33, an environmentally friendly working fluid, in smooth surfaces open microchannels (SOM) and multi-stage enhanced open microchannels (MSEOM). The study investigated the flow boiling performance of SF-33 in two open microchannels with different mass fluxes, heat fluxes and inlet subcooling. The different boiling flow patterns in two open microchannels were analyzed with high-speed visualization. The results indicated that under different inlet cooling conditions, the MSEOM improved the flow boiling heat transfer performance significantly and improved the wall temperature uniformity. The visualization images indicated that the flow pattern transition was different due to the different surface structures of two open microchannels. The SOM experienced plug-stratified flow at high heat flux, while MSEOM transformed into churn flow due to its sintered copper powder and wire mesh micro-nanocomposite structures. The multi-stage enhanced structures exhibited excellent hydrophilicity and wicking capability, leading to a less pressure drop in the MSEOM compared with that in SOM. The pressure drops for both open microchannels decreased as inlet subcooling increased.

Numerical analysis of enhanced heat transfer and nanofluid flow mechanisms in fan groove and pyramid truss microchannels

A combined groove and truss structure is designed for rectangular microchannel heat sinks with 4 % Al2O3 nanofluid as the working fluid to address the heat dissipation requirements of high heat flux density electronic devices. The effects of fan shape, elliptical, waterdrop, rectangular, trapezoidal and triangular grooves on the heat transfer characteristics and mechanical properties of microchannels are investigated. The fan-shaped groove microchannels with the best overall heat transfer performance and excellent mechanical properties. The stress of the fan-shaped grooved truss microchannel is reduced by 76.41 % compared to the smooth microchannel. Three structural parameters were investigated, including the length of the truss in the spreading direction (Lx), the ratio of truss flow downstream length to upstream length (e) and truss rod diameter (d). The performance of the microchannels is reflected by the integrated heat transfer factor and the field coordination number. The individual structural parameters were analysed in a single-factor comparison. The microchannels exhibited the best hydrothermal performance in the Reynolds number range of 500 ∼ 1300 at Lx = 0.8 mm, e = 3, d = 0.2 mm. When the Reynolds number is 900, the microchannel with the optimal parameter combination exhibits a remarkable enhancement of 234 % in the Nusselt number and an 80 % increase in the integrated heat transfer factor compared to the rectangular microchannel.

An experimental study on flow boiling heat transfer in porous-ribbed micro-channels

Experiments were carried out to investigate flow boiling heat transfer characteristics in a micro-channel heat sink with porous ribs at three rib-heights (i.e. Hrib = 200 μm, 400 μm, and 600 μm), three mass fluxes (i. e. 200 kg/m2s, 300 kg/m2s, and 400 kg/m2s), and a range of wall heat fluxes (30–1300 kW/m2). The heat transfer coefficients and temperature uniformity in porous-ribbed micro-channels under different conditions were analyzed and compared to the smooth micro-channels (i.e. smooth flat surface and smooth-ribbed surfaces). The flow regimes in both porous and smooth micro-channels were visualized under typical flow boiling conditions. The results showed that micro-channels with higher rib heights experience an earlier transition to the critical heat flux (CHF) region due to the narrower flow path. Compared to the smooth micro-channel, the configuration of porous coating significantly enhances the heat transfer in the micro-channel without ribs or with lower rib height (Hrib = 200 μm) due to the strong hydrophilicity, good liquid storage, and rewetting ability on porous surfaces. At G = 400 kg/m2s, the porous-flat surface achieves a 67.9 % enhancement of heat transfer coefficient in the micro-channel compared to the smooth-flat micro-channel. However, with a higher rib height (Hrib = 400 μm and 600 μm), the thermal resistance is increased which hinders the heat conduction along the rib-height direction. A significant superheat is observed at the bottom of porous surface, thereby deteriorating the heat transfer performance in the porous ribbed micro-channel.

Critical heat flux at high mass velocities in a microchannel and surface tension measurements for R514a

This paper presents pioneer experimental data on critical heat flux (CHF) during flow boiling of low-pressure fluid R514a in a microchannel. The authors conducted experiments in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm, a saturation temperature range of 56 to 122.7°C, and a mass velocity range of 1500 to 7500 kg/m2s. This research achieved high critical heat flux values, up to 1.85 MW/m2. The results were compared with the CHF of R123 under similar conditions and a higher CHF for R514a was observed. The performance of CHF predictive methods against the experimental data was performed. Since predictive methods for the CHF in microchannels need surface tension data, but there is a lack of experimental information about this property for R514a in the literature, this paper also presents experimental results for the surface tension of R514a and an expression for calculation of this property as a function of the temperature.

Thermal-hydraulic performance of various designs of microchannel heat sink with internal bifurcations

Microchannel Heat Sinks (MCHS) can dissipate large amounts of heat in a compact area making them a main choice for managing heat in space-limited applications. Recently, with the help of 3D metal printing, it became easy to fabricate various designs of MCHS with internal complex designs. This study, compare the thermo-hydraulic performance of three bifurcation based MCHS designs, with the traditional straight MCHS. Single-phase cooling of surfaces with high heat fluxes of 200 kW/m2 and 400 kW/m2, was numerically analyzed. The model is validated with the literature. The MCHS designs under study included: a standard smooth microchannel (Case A), a channel with a single extended bifurcation (Case B), multiple inline bifurcations (Case C), and a design employing stepwise bifurcations (Case D). Case A served as the reference case for comparison. Results showed that incorporating bifurcations substantially enhances the MCHS's heat removal efficiency. Specifically, the increment in Nusselt number for Cases B, C, and D compared to case A were 1.78, 1.6, and 1.55, respectively at heat flux of 200 kW/m2 and Reynolds of 200. Moreover, MCHS designs with bifurcations significantly improved temperature uniformity, achieving the best temperature uniformity of 7.7 °C at a Reynolds number of 700 under a 400 kW/m2 heat flux.

Thermal and hydraulic performance of Al alloy-based 3D printed triangular microchannel heatsink governed by rough walls with graphene and alumina nanofluids as working liquid

Microchannel heat sinks (MCHS) are known for providing enhanced cooling performance but their fabrication requires complex and multi-step processes. The recent development of additive manufacturing has enabled the fabrication of state-of-art monolithic structures that had been impossible to build using conventional methods. In this work, a monolithic cross-flow triangular cross-section MCHS was fabricated from aluminum alloy (AlSi10Mg) using the Direct Metal Laser Sintering (DMLS) process. The microchannel wall surface roughness was measured and the cross-section shrinkage of the microchannels was compared with the initial design hydraulic diameter of 500 µm–1000 µm. The MCHS with an initial design hydraulic diameter of 750 µm possessed a relative wall surface roughness, R a of 7.7%. The triangular cross-section hydraulic diameter underwent a shrinkage of 15.2% and 5.3% in terms of the reduction in angle between adjacent side alloys. Experiments were conducted for Reynolds numbers between 50 and 275 with nanofluids containing graphene and Al2O3 nanoparticles in water/water +10% ethylene glycol; these were compared with their respective base fluids. The Poiseuille number indicated that flow was laminar developed with base fluid and laminar developing with nanofluid as coolant. Despite providing the lowest thermal resistance, the graphene nanoparticles in water created the greatest pressure drop leading to a reduced performance coefficient. Al2O3 nanoparticles in water/water +10% ethylene glycol were found to have 7.7% and 20% better performance coefficients than their respective base fluids.

Critical heat flux, heat transfer and pressure drop at high mass fluxes for R123 in a single microchannel

This paper presents new experimental data on pressure drop, heat transfer coefficient, and critical heat flux during flow boiling of low-pressure fluid R123 in microchannels. High mass velocities were used, above the values found in the literature in tests with this fluid. Experiments were conducted in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm. The mass velocities used were between 1500 and 7500 kg/m2s. Saturation temperature ranged from 40.1 to 85.1 oC. The outlet vapor quality varied between -0.54 and 0.53 for the CHF results, between 0 and 0.5 for the pressure drop results, and between 0 and 0.35 for the HTC results, respectively. Record critical heat flux values for flow boiling of R123 in smooth microchannels were achieved, reaching 1.44 MW/m2. An analysis was made of the influence of an upstream pressure drop in avoiding instabilities during critical heat flux experiments. The highest heat transfer coefficient values during convective boiling of R123 in smooth microchannels were also obtained, with values close to 43.3 kW/m2K. The saturation temperatures used were among the highest for flow boiling of R123 in smooth microchannels found in the literature. Correlations for frictional pressure drop, heat transfer coefficient, and critical heat flux were evaluated in predicting the experimental data. An analysis was made on the influence of the void fraction correlation when estimating both the accelerational and frictional pressure drop.

RELATIVE PERMEABILITY MODEL OF TWO-PHASE FLOW IN ROUGH CAPILLARY ROCK MEDIA BASED ON FRACTAL THEORY

In this paper, the gas-water two-phase flow characteristics of rock media are studied based on fractal theory and the relative roughness model, and the analytical model of gas-water relative permeability of rock pores with relative roughness is derived. Through numerical simulation, it is found that the maximum flow velocity in the rough microchannel is greater than the maximum flow velocity in the smooth microchannel, but the average flow velocity in the rough microchannel is less than the average flow velocity in the smooth microchannel. The theoretical model results show that the dimensionless permeability of gas-water two-phase flow is inversely proportional to the fractal dimension of relative roughness and tortuosity, and the relative permeability of gas-water two-phase flow is inversely proportional to the relative roughness. The correctness of the model is verified by comparing it with the relevant experimental data.

Optically smooth microchannels in the volume of lithium niobate fabricated by selective etching of fs-laser written structures and subsequent annealing

3D-hollow microstructures with few tens of micrometer in diameter with an optical-quality surface roughness (Ra ≤ 1 nm) have been fabricated in the volume of lithium niobate by selective etching of fs-laser written structures and post-etching annealing. The fs-laser writing parameters and the annealing process have been refined to reduce the average surface roughness and the shape change. Systematically investigating the annealing process, an empirical functional description of the temporal evolution of the surface roughness was found completing the data set of processing parameters for selective etching of fs-laser written structures, allowing to control the fabrication process of the hollow microstructures concerning both shape and surface roughness. Thus, our results represent another milestone within the research towards monolithic micro-(opto)fluidic applications inside the multifunctional crystal lithium niobate.

Heat transfer and flow analyses of microchannel heat sink with twisted blade-like fins

To enhance heat transfer coefficient, decrease pressure penalty, and improve wall temperature uniformity, a twisted blade-like fin with low-drag airfoil section is proposed in this study. The flow and heat transfer performances of microchannels with the fins are numerically investigated at Re = 50–700. Results show that the novel fins improve the heat transfer (Nu/Nu 0) significantly with a small pressure penalty (f/f 0), and thus show an excellent comprehensive thermal performance (TP). The ranges of Nu/Nu 0, f/f 0 and TP are 1.16–4.76, 1.51–2.36, and 1.01–3.58, respectively. The TP of TAMCHS reaches 3.58, exhibiting an increase of 46.7, 37.7, 22.6, and 16.6% compared to four models in previous study. The reduced pressure penalty is attributed to the streamlined configuration, and the improved heat transfer coefficient is attributed to the normalwise secondary flow stimulated by the twisted fins, which enhances heat transfer in the upper portion of the channel. Compared to a smooth microchannel heat sink (SMCHS), the major advantages lie in the significant reduction of sidewall temperatures and notable improvement in temperature uniformity on the top and bottom walls. The average and maximum wall temperature are reduced by 48.1 and 49.0 °C at most, respectively, and the greatest decline of wall temperature gradient is 55.0 °C.

Numerical study on flow and heat transfer in a novel symmetric sinusoidal wavy microchannel heat sink with rectangular rib prisms

To enhance thermal and overall performances, a novel microchannel heat sink (SSW-RP) was devised by integrating symmetric sinusoidal wavy sidewalls with rectangular rib prisms within the channels. A numerical comparative study was conducted to explore heat transfer mechanism in channels by investigating the flow and heat transfer characteristics of SSW-RP in comparison with those of smooth straight microchannel (MC), smooth straight microchannel with rectangular rib prisms (MC-RP), and symmetric sinusoidal wavy microchannel (SSW). The results show that the addition of rib prisms makes the fluid flow into wave peaks, reducing and even eliminating vortices from wave peaks of the symmetric sinusoidal wavy microchannel, thus significantly improving heat transfer performance. The new design SSW-RP had the highest relative Nusselt number, which reaches 4.16 at Re = 800. Further, the hydrothermal performance sensitivity of SSW-RP to the relative width (α) and length (β) of rib prisms is studied, and the best overall performance factor of SSW-RP finally can be achieved up to 1.88 at α = 0.35, β = 0.4 and Re = 800.

Heat transfer enhancement and temperature uniformity improvement of microchannel heat sinks with twisted blade-like fins

To enhance heat and fluid exchange between main flow and near-wall flow in microchannels, a twisted blade-like fin with an advantage in stimulating both spanwise and normalwise secondary flow is proposed. The cross sections of the fin are low-drag airfoils in different orientations. The flow and heat transfer performances of microchannels with fins are numerically investigated at Re = 50–700. The results show that the twisted blade-like fin improves heat transfer process significantly, especially wall temperature uniformity, and reduces the flow separation region behind the fin, resulting in an obvious small pressure penalty. At small inflow (Re = 50), the best heat transfer and lowest pressure penalty are obtained in the microchannel with a single fin. When Re > 150, better heat transfer is obtained in the microchannel with three fins, and the highest comprehensive thermal performance (TP) reaches 3.58. The twisted direction of fins has a significant effect on heat transfer but less on flow drag. Due to the twisted fins, left and right walls experience an overall decline in temperature, and the temperature uniformity of top and bottom walls is improved. Compared with a smooth microchannel, the average and maximum temperature of the investigated microchannels are reduced by 48.1 K and 49.0 K at most respectively.

Enhanced thermo-hydraulic performance with two-phase flow boiling in wavy microchannel heat sink – A numerical investigation

This article demonstrates that using a wavy microchannel instead of a smooth microchannel can enhance thermal and hydraulic performance in both single and two-phase flow. Various performance parameters such as Nusselt number (Nu), friction factor (f), performance factor (PF), and enhancement factor (EF) are estimated for different wavy microchannels with varying waviness and amplitudes. The two-phase heat transfer coefficient (HTC) varies significantly with heat flux, particularly due to nucleate boiling. The heat transfer improvement with a wavy microchannel is evident with the highest values of the Nusselt number ratio (Nutp/Nusp = 2.35), PFwavy = 1.4, and EF = 1.88 achieved at specific operating conditions (heat flux = 100 W/cm2, mass flux = 188 kg/m2s, amplitude = 0.1 mm, wavelength = 1 mm). However, as mass flux, heat flux, and amplitude increase and wavelength decrease, the Nusselt number ratio Nutp/Nusp, PF, and EF decline due to the pressure drop penalty outweighing the benefits of heat transfer enhancement. The article also explores different designs of wavy microchannels for mitigating hot spots, including one with a decreasing wavelength along the flow direction and another with a shorter wavelength in the middle. However, these designs are more suitable for single-phase flow.

Etude numérique de l’effet des générateurs de vortex longitudinaux sur le transfert thermique d’un écoulement laminaire traversant un micro-canal

We conducted, in this work, a three-dimensional numerical study of forced convection heat transfer of a laminar flow of water passing through micro-channels with and without longitudinal vortex generator, using the CFD code “Ansys- Fluent”. This work aimed to elucidate the effect of vortex generators on the dynamic and thermal behavior of the micro-fluidic flow. The results obtained show that the increase in the Reynolds number leads to an improvement in the quality of heat transfer in both cases of the study. Rectangular micro-channel with LVG can improve heat transfer compared to smooth rectangular micro-channel while consuming more pressure drop. In the range of Reynolds numbers between 200 and 1200, a 2-21% increase in the mean Nusselt number was observed for micro-channels with LVG compared to smooth micro-channels. This occurs through better mixing of the fluid, reduction in the thickness of the thermal boundary layer, and increased heat transfer area. In addition, the friction factor has been increased by more than 50% compared to smooth micro-channels, due to the local resistance of LVGs and the presence of secondary flows.

Experimental investigation into flow boiling heat transfer and pressure drop in porous coated microchannels

The flow boiling heat transfer and pressure drop in porous coated micro-channels were experimentally investigated at four porous coating particle sizes (i.e. 75 μm, 48 μm, 25 μm, and 13 μm), three mass flow rates (150 kg/m2s, 275 kg/m2s, and 400 kg/m2s), and a range of wall heat fluxes (0–1285 kW/m2). The comprehensive heat transfer (CHT) performance in the porous micro-channels was obtained and compared to the smooth micro-channel. The flow patterns in the micro-channels were visualized to analyze the flow boiling heat transfer characteristics near wall region. The results show that the heat transfer coefficient and critical heat flux were significantly enhanced and the pressure drop was increased in porous coating micro-channels as compared to the smooth micro-channel. As the size of sintered particle increases, the heat transfer coefficient on porous coating surface is increased, while the pressure drop in the micro-channel is increased. Due to its good liquid storage and re-wetting ability, the porous coating surface has a higher hydrophilicity, resulting in a higher critical heat flux in the porous micro-channel than the smooth channel. However, the CHT performance of the porous coating surface is not always better than the smooth surface. Among the studied porous-coating surfaces, the micro-channel with the averaged sintered particle size 13 μm has the best CHT, while the surface with 75 μm sintered particles has the highest critical heat flux and heat transfer coefficient. Compared to the smooth surface, the critical heat flux and heat transfer coefficient on the 75 μm porous coating surface were enhanced by 53.2 % and 114.5 %, respectively.

Influence of obstruction’s unilateral length on flow and heat transfer performance of micro-channel heat sinks with flow obstructions

As the density and integration of electronic equipment continue to increase, it becomes necessary to build micro-channel heat sinks with more efficient cooling effects to solve the heat removal issue. This paper proposes a series of obstructions established on a smooth water-cooled micro-channel and analyzes the heat transfer and flow performance from the viewpoint of fluid dynamics. By adjusting the unilateral side length of the obstructions, the simulation conducted a comparative analysis between the seven different obstruction shapes and smooth micro-channel heat sinks, focusing on Nusselt number, thermal resistance, and pressure loss across a range of Reynolds numbers. The findings demonstrate that the heat dissipation capacity of micro-channel heat sinks with obstructions is consistently superior to that of the smooth heat sink, and the performance of these heat sinks is affected by the length of unilateral side of the obstructions. With no consideration of pumping power, the paper argues that the heat sink with obstructions, specifically those with a unilateral side length of 0.06 mm, has the best heat dissipation capacity at Reynolds numbers below 460. At this Reynolds number, the Nusselt number increases by 99% and the thermal resistance decreases by 33% compared to a smooth micro-channel heat sink. However, when the pumping power is considered, the heat sink with obstructions whose unilateral sides are the shortest has the best performance due to its minimal pressure loss. While meeting the thermal resistance requirement of 0.033, it requires only one-fourth of the pumping power needed by a heat sink with obstructions whose unilateral sides are longest.

Thermal-hydraulic performance and multi-objective optimization using ANN and GA in microchannels with double delta-winglet vortex generators

This study presents a multi-objective optimization approach to design microchannels with double delta-winglet vortex generators for obtaining the best hydrothermal performance under high heat flux. Grey relational analysis, neural networks, and Non-dominated Sorting Genetic Algorithm (NSGA-II) are utilized in the optimization process. Back-propagation neural network improved by genetic algorithm (GA-BP) is found to have superior generalization and prediction abilities for Nusselt number (Nu) and friction factor (f) compared to Back-propagation neural network (BPNN). The design parameters include the angle between vortex generator and bottom plane (θ1), the angle between the baseline of vortex generator and water incoming flow direction (θ2), the distance from the apex of the vortex generator to the baseline (h), and inlet velocity (vin). NSGA-II is employed to obtain the optimal design point (θ1 = 90°, θ2 = 45°, h = 0.3 mm, vin = 0.8 m/s). The hydrothermal performance under this design point is significantly improved, as compared to smooth microchannel, with a 128.1% increase in Nu, a 72.0% increase in f, and a performance evaluation criterion (PEC) of 1.89. Furthermore, flow and temperature fields, entropy production, local Nu, local f and axial vorticity are analyzed. The results highlight the potential of double delta-winglet vortex generators to improve overall thermal performance in high heat flux applications.