Mesoscale investigation of frost formation on a cold surface

Frost formation on a horizontal flat copper surface was experimentally investigated using microscopic observations. The experiments were carried out on −20 to 0 °C copper surfaces with 22 °C air and 15–85% relative humidities. The experiments showed that the frost formation on a cold surface generally begins with the formation and growth of condensate droplets, freezing of the super-cooled condensate droplets, formation and growth of initial frost crystals on the frozen droplets, growth of frost crystals accompanied by the collapse of some of the crystals, and finally frost layer growth. The freezing onset time and diameter of the super-cooled condensate droplets were characterized. The initial frost crystals can be classified into four groups according to their appearance and shape, with the variations of the frost crystal shape as a function of the cold surface temperature and air humidity.     

Effects of heat flux,mass flux,vapor quality,and saturation temperature on flow boiling heat transfer in microchannels

Flow boiling heat transfer with the refrigerants R-134a and R-245fa in copper microchannel cold plate evaporators is investigated. Arrays of microchannels of hydraulic diameter 1.09 and 0.54 mm are considered. The aspect ratio of the rectangular cross section of the channels in both test sections is 2.5. The heat transfer coefficient is measured as a function of local thermodynamic vapor quality in the range −0.2 to 0.9, at saturation temperatures ranging from 8 to 30 °C, mass flux from 20 to 350 kg m⁻² s⁻¹, and heat flux from 0 to 22 W cm⁻². The heat transfer coefficient is found to vary significantly with heat flux and vapor quality, but only slightly with saturation pressure and mass flux for the range of values investigated. It was found that nucleate boiling dominates the heat transfer. In addition to discussing measurement results, several flow boiling heat transfer correlations are also assessed for applicability to the present experiments.     

An analysis of incipient frost formation

An analytical and experimental study of frost growth on a cooled plate being exposed to forced convective humid air stream was carried out. Attention is mainly focused on the incipient phenomena of the frost formation including the growth of supercooled water droplets which are based on condensation of water vapor leaving the air. Two kinds of the plates having different contact angles to water droplet (110 deg and 43 deg) are used as the testing ones. The effects of the velocity, humidity, and temperature of the air stream, the contact angle to water droplet, and the temperature of the cooled plate on the frost formation are extensively determined. An analytical model which is based on the experimental observations is found to closely predict the general trends in the growths of the supercooled water droplets and the porous frost layer.     

A study of frost growth and densification on flat surfaces

The present study advances a theoretical and experimental investigation of the frost growth and densification on flat surfaces. This study focuses on the most important factors affecting the frost formation process, i.e. the surrounding air temperature, humidity and velocity, and the surface temperature. The processes of frost growth and densification were investigated experimentally in order to provide a physical basis for the development of a theoretical model to predict the variation of the frost layer thickness and mass with time. The mathematical model was based on mass and energy balances within the frost layer, assuming the frost as a porous medium and accounting for the supersaturation of the moist air on the frost surface. The governing equations for mass and heat diffusion were integrated analytically, giving rise to a semi-algebraic formulation which requires numerical integration of only one time dependent ordinary differential equation. When compared with experimental data, the model predictions of the frost thickness as a function of time agreed to within ±10% error bands. The experimentally-validated model was then used to predict the frost layer growth and densification with respect to the operation conditions such as plate surface temperature, air stream temperature, humidity and velocity.     

An analytical study of frost nucleation and growth during the crystal growth period

An analytical study was made to clarify the fundamental nature of the early stage of crystal growth period of frost formation phenomena. A suitable model was developed by using the principles of crystallization and nucleation theory. The effect of four dominant parameters of frost formation; plate temperature, air temperature, air humidity ratio and Reynolds number, was studied. Ice crystal density variation with temperature reported by cloud physicists is used in the model to predict the density variation of frost during the crystal growth period. The temperature variation in the frost layer is formulated and vapor diffusion through the frost layer is taken in the consideration.     

Hydrodynamics and boiling phenomena of water droplets impinging on hot solid

The collision of single water droplets with a hot Inconel 625 alloy surface was investigated by a two-directional flash photography technique using two digital still cameras and three flash units. The experiments were conducted under the following conditions: the pre-impact diameters of the droplets ranged from 0.53 to 0.60 mm, the impact velocities ranged from 1.7 m/s to 4.1 m/s, and the solid surface temperatures ranged from 170 °C to 500 °C. When a droplet impacted onto the solid at a temperature of 170 °C, weak boiling was observed at the liquid/solid interface. At temperatures of 200 or 300 °C, numerous vapor bubbles were formed. Numerous secondary droplets then jetted upward from the deforming droplet due to the blowout of the vapor bubbles into the atmosphere. No secondary droplets were observed for a surface temperature of 500 °C at the low-impact Weber numbers (∼30) associated with the impact inertia of the droplets. Experiments using 2.5-mm-diameter droplets were also conducted. The dimensionless collision behaviors of large and small droplets were compared under the same Weber number conditions. At temperatures of less than or equal to 300 °C, the blowout of vapor bubbles occurred at early stages for a large droplet. At a surface temperature of 500 °C, the two dimensionless deformation behaviors of the droplets were very similar to each other.     

Thermo-hydraulic characterization of a louvered fin and flat tube heat exchanger

In the present study, a whole heat exchanger with a hydraulic diameter of 2.3 mm is tested, which is a minichannel heat exchanger according to the Kandlikar classification. This is a louvered fin and flat tube heat exchanger currently used in car cooling systems, also known as radiator. A glycol-water mixture (60/40 in volume) circulates through the tubes at flows ranging from 100 to 7800 l/h and at a supply temperature of 90 °C. This fluid is cooled with ambient air at a temperature of 20 °C and at frontal air velocities varying between 0.5 and 7 m/s. The thermohydraulic performance of the heat exchanger is compared with the classical correlations given in the literature for the heat transfer and the friction factor calculation. On the glycol-water side the heat exchanger is characterized for Reynolds numbers from 30 to 8000. A first comparison is carried out with the correlations available in the literature with a purely predictive model by obtaining a predictive value with a systematic under prediction lower than 10%. In a second step a semi-empirical model is considered to identify the experimental heat transfer coefficients for this application.     

Three phase liquid–liquid–gas flows in 5.6 mm and 7 mm inner diameter pipes

Three phase liquid–liquid–gas flow maps in pipes of medium inner diameters (5.6 mm and 7 mm), are presented. A low viscosity paraffin oil (4.5 × 10⁻³ Pa s viscosity and 818.5 kg m⁻³ density at 20 °C), deionised water and air are flowing concurrently in Schott Duran^® glass pipes. A decreasing pipe diameter changes the flow pattern maps and also the behavior of the transition boundaries. Flow patterns are determined by high speed photography. To illuminate the pipe, laser induced fluorescence (LIF) is applied. The laser sheet is cutting through the axial vertical plane of the pipe. The laser light excites a fluorescent dye (uranine) in the water phase to separate the phases optically. The resulting flow maps are compared with literature data and a theoretical model.     

Experimental study on condensation heat transfer in vertical minichannels for new refrigerant R1234ze(E) versus R134a and R236fa

Experimental condensation heat transfer data for the new refrigerant R1234ze(E), trans-1,3,3,3-tetrafluoropropene, are presented and compared with refrigerants R134a and R236fa for a vertically aligned, aluminum multi-port tube. Local condensation heat transfer measurements with such a multi-microchannel test section are very challenging due to the large uncertainties related to the heat flux estimation. Presently, a new experimental test facility was designed with a test section to directly measure the wall temperature along a vertically aligned aluminum multi-port tube with rectangular channels of 1.45 mm hydraulic diameter. Then, a new data reduction process was developed to compute the local condensation heat transfer coefficients accounting for the non-uniform distribution of the local heat flux along the channels. The condensation heat transfer coefficients showed the expected decrease as the vapor quality decreased (1.0-0.0) during the condensation process, as the mass velocity decreased (260-50 kg m⁻² s⁻¹) and as the saturation temperature increased (25-70 °C). However, the heat transfer coefficients were not affected by the condensing heat flux (1-62 kW m⁻²) or by the entrance conditions within the tested range. It was found that the heat transfer performance of R1234ze(E) was about 15-25% lower than for R134a but relatively similar to R236fa. The experimental data were then compared with leading prediction methods from the literature for horizontal channels. In general, the agreement was poor, over-predicting the high Nusselt number data and under-predicting the low Nusselt number data, but capturing the mid-range quite well. A modified correlation was developed and yielded a good agreement with the current database for all three fluids over a wide range of operating conditions.     

Experimental studies of adiabatic flow boiling in fractal-like branching microchannels

Experimental results of adiabatic boiling of water flowing through a fractal-like branching microchannel network are presented and compared to numerical model simulations. The goal is to assess the ability of current pressure loss models applied to a bifurcating flow geometry. The fractal-like branching channel network is based on channel length and width ratios between adjacent branching levels of 2^−1/2. There are four branching sections for a total flow length of 18 mm, a channel height of 150 μm and a terminal channel width of 100 μm. The channels were Deep Reactive Ion Etched (DRIE) into a silicon disk. A Pyrex disk was anodically bonded to the silicon to form the channel top to allow visualization of the flow within the channels. The flow rates ranged from 100 to 225 g/min and the inlet subcooling levels varied from 0.5 to 6 °C. Pressure drop along the flow network and time averaged void fraction in each branching level were measured for each of the test conditions. The measured pressure drop ranged from 20 to 90 kPa, and the measured void fraction ranged from 0.3 to 0.9. The measured pressure drop results agree well with separated flow model predictions accounting for the varying flow geometry. The measured void fraction results followed the same trends as the model; however, the scatter in the experimental results is rather large.     

Thermo-mechanical response of Al 6061 with and without equal channel angular pressing (ECAP)

The thermo-mechanical responses of Al 6061 before and after equal channel angular pressing (ECAP) at different strain rates and temperatures were measured. Al 6061 was solution heat treated before ECAP pressing at room temperature and subjected to up to three passes. After pressing, the billets were aged at 100 °C for 2 days. An as-received Al 6061-T651 was studied similarly to investigate the differences between processed and non-processed specimens. The responses of ECAP material were determined at −30, 22, 125 and 250 °C, and at strain rates from 10⁻⁵ to 2530 s⁻¹; the 6061-T651 specimens were subjected to uniaxial compressive loading at −31, 22, 85, 150, 230 and 315 °C, and strain rates ranging from 10⁻⁵ to 2200 s⁻¹. It was found that, the ECAP process increases the strength versus the T651 condition. Additionally, the Al 6061 ECAP is not sensitive to strain rate at room and lower temperatures, but the sensitivity increases as the number of passes and/or temperature are increased and this is the same for the non-processed material. Increasing the number of passes increases the flow stress at room and lower temperatures, has almost no effect at 125 °C and decreases at 250 °C. For both materials, the dynamic flow stress is higher than the stress at quasi-static strain rates even when the quasi-static strain rate regime is insensitive to strain rate. The Al 6061 has strong texture after one pass but steadily increases as the number of passes are increased. This is the first study that reports on the thermo-mechanical responses of ECAP and non-ECAP Al 6061 at such a wide range of strain rates, including dynamic, and temperatures.     

Evaporation heat transfer coefficient in single circular small tubes for flow natural refrigerants of C3H8, NH3, and CO2

An experimental study of evaporation heat transfer coefficients for single circular small tubes was conducted for the flow of C₃H₈, NH₃, and CO₂ under various flow conditions. The test matrix encompasses the entire quality range from 0.0 to 1.0, mass fluxes from 50 to 600 kg m⁻² s⁻¹, heat fluxes from 5 to 70 kW m⁻², and saturation temperatures from 0 to 10 °C. The test section was made of circular stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 2000 mm in a horizontal orientation. The test section was uniformly heated by applying electric power directly to the tubes. The effects of mass flux, heat flux, saturation temperature, and inner tube diameter on the heat transfer coefficient are reported. Among the working refrigerants considered in this study, CO₂ has the highest heat transfer coefficient. Laminar flow was observed in the evaporative small tubes, and was considered in the modification of boiling heat transfer coefficients and pressure drop correlations.     

Fins and turbulence promoters for heat transfer enhancement in latent heat storage systems

One of the serious problems associated with the operation of PCM storage system is the heat transfer in and out of the element containing the PCM. This paper presents the results of an experimental investigation of the effects of radial fins and turbulence promoters on the enhancement of phase change heat transfer external to a horizontal tube submersed in the PCM with the working fluid flowing through it. The experimental measurements were realized on a bare cupper tube and an identical cupper tube fitted with radial fins. The fins investigated are 40, 60, 120 and 180 mm diameters. A turbulence promoter made of stainless steel wire of 1.0 mm diameter coiled in a helical form with a pitch of 25.0 mm was inserted into the cupper tubes. The tests were realized on bare tubes, finned tubes and finned tubes with the turbulence promoter inserted into the finned tubes. The measurements were realized for the working fluid temperatures in the range of −10 °C, to −25 °C and six values of the mass flow rate ranging from 0.013 to 0.031 kg/s. The position of the phase interface was photographed by a high resolution digital camera and scanned to determine the real interface position by comparison with a precision measuring scale. The results of the phase interface position, velocity of the interface, solidified mass fraction and the time for complete solidification are presented in function of the working fluid temperature, the working fluid mass and the tube arrangements. The results are presented and discussed.     

Progressive fracture of laminated composite stiffened plate

Laminated fiber-reinforced composite stiffened plate with [0/90/±45]_S plies made of S-Glass/epoxy are evaluated via computational simulation to study damage and fracture progression. The loads are pressure and temperature which varies from 21 to 65.5 °C (case I) and from 143.3 to 21 °C (case II). An integrated computer code is used for the simulation of the damage progression. Results show that damage initiation begins at low load level, with matrix cracking at the 0° (bottom and top) plies, fiber fracture at the bottom (0°) ply and interply delamination at the top (0°) ply. Increasing the applied pressure, the damage growth is expended resulting in fracture through the thickness of the structure. At this stage, 90% of the plies damage at applied pressure 15.306 MPa for the case I and 15.036 MPa for the case II. After this stage, the cracks propagate rapidly and the structure collapses.     

Turbulent mixed convection flow over a backward-facing step––the effect of the step heights

Effect of the backward-facing step heights on turbulent mixed convection flow along a vertical flat plate is examined experimentally. The step geometry consists of an adiabatic backward-facing step, an upstream wall and a downstream wall. Both the upstream and downstream walls are heated to a uniform and constant temperature. Laser–Doppler velocimeter and cold wire anemometer were used, respectively, to measure simultaneously the time-mean velocity and temperature distributions and their turbulent fluctuations. The experiment was carried out for step heights of 0, 11, and 22 mm, at a free stream air velocity, u_∞, of 0.41 m/s, and a temperature difference, ΔT, of 30 °C between the heated walls and the free stream air. The present results reveal that the turbulence intensity of the streamwise and transverse velocity fluctuations and the intensity of temperature fluctuations downstream of the step increase as the step height increases. Also, it was found that both the reattachment length and the heat transfer rate from the downstream heated wall increase with increasing step height.     

Influence of the aspect ratio on boiling flows in rectangular mini-channels

This article presents experiments conducted with two single rectangular mini-channels of same hydraulic diameter (1.4 mm) and different aspect ratios for conditions of horizontal boiling flow. The Forane® 365 HX used was subcooled (ΔT_sub = 15 °C) for all the boiling curves presented in the paper. Local heat transfer coefficients were measured for heat flux ranging from 25 to 62 kW m⁻² and mass flux from 200 kg m⁻² s⁻¹ to 400 kg m⁻² s⁻¹. The boiling flows were observed with two different cameras (depending on the flow velocity) through a visualization window. The flow patterns in the two channels were compared for similar conditions. The results show that the boiling heat transfer coefficient and the pressure drop values are different for the two single mini-channels. For low heat flux condition, the channel with lowest aspect ratio (H/W = 0.143) has a higher heat transfer coefficient. On the other hand, for high heat flux condition, the opposite situation occurs, namely the heat transfer coefficient becomes higher for the channel with highest aspect ratio (H/W = 0.43). This is probably due to the earlier onset of dryout in the channel with lowest aspect ratio. For the two cases of heating, the pressure drop for the two-phase flow remains lower for the channel with lowest aspect ratio. These results show that the aspect ratio plays a substantial role for boiling flows in rectangular channels. As for single-phase flows, the heat transfer characteristics are significantly influenced (even though the hydraulic diameter remains the same) by this parameter.     

Experimental studies of a steam jet refrigeration cycle: Effect of the primary nozzle geometries to system performance

This paper describes an experimental investigation of a steam jet refrigeration. A 1 kW cooling capacity experimental refrigerator was constructed and tested. The system was tested with various operating temperatures and various primary nozzles. The boiler saturation temperature ranked from 110 to 150 °C. The evaporator temperature was fixed at 7.5 °C. Eight primary nozzles with difference geometries were used. Six nozzles have throat diameters ranked from 1.4 to 2.6 mm with exit Mach number of 4.0. Two remained nozzles have equal throat diameter of 1.4 mm but difference exit Mach number, 3.0 and 5.5. The experimental results show that the geometry of the primary nozzle has strong effects to the ejector performance and therefore the system COP.     

Flow regimes and two-phase pressure gradient in horizontal straight tubes: Experimental results for HFO-1234yf, R-134a and R-410A

Two-phase flow regime visualizations of HFO-1234yf and R-134a in a 6.70 mm inner diameter glass straight tube have been simultaneous investigated by top and side views with a high speed high resolution camera. No major difference was observed between both refrigerants. HFO-1234yf flow regimes were satisfactorily predicted by the Wojtan et al. [1] flow pattern map. In addition, 819 pressure drop data points measured during two-phase flow of refrigerants HFO-1234yf, R-134a and R-410A in horizontal straight tubes are presented. The tube diameter (D) varies from 7.90 to 10.85 mm. The mass velocity ranges from 187 to 1702 kg m⁻² s⁻¹ and the saturation temperatures from 4.8 °C to 20.7 °C. The results are compared against 10 well-known two-phase frictional pressure drop prediction methods. For the entire database, the best accuracy is given by the method of Müller-Steinhagen and Heck [2] with around 90% of the data predicted within a ±30% error band. An analysis was carried out on the maximum pressure gradient and on the corresponding vapor quality. A statistical analysis for each flow regime was also carried out.     

Frost formation on a vertical plate in simultaneously developing flow

It is well recognized that frost formation on heat exchanger surfaces seriously affects the performance of a refrigeration system. Consequently, defrosting is essential, yet it is only effective when both analytical tools and comprehensive experimental data on frost formation are available. In air conditioning units, frost formation most commonly takes place in the entrance region of the heat exchanger. Therefore, in this study, an experimental investigation was undertaken to characterize the effect of environmental conditions on the frost growth occurring on a vertical plate in the hydrodynamically and thermally developing region. Several experiments were performed while four environmental parameters, inlet air temperature, inlet air humidity, air velocity and cooling surface temperature, were varied. The thickness, mass, and density of the frost layer were determined from the measured data and empirical correlations were reduced from dimensionless parameters.     

Parametric studies and effect of scale-up on wall-to-bed heat transfer characteristics of circulating fluidized bed risers

In the present work a comparative study of steady state wall-to-bed heat transfer was conducted along the risers of height 2.85 m of three different circulating fluidized beds (CFBs) with bed cross sections of 0.15 m × 0.15 m, 0.20 m × 0.20 m, and 0.25 m × 0.25 m, respectively. Experiments were conducted on each CFB unit for five superficial air velocities (U = 2.5 m/s, 2.75 m/s, 3 m/s, 3.3 m/s, and 4 m/s) and two different weights of sand inventory per unit area of the distributor plate (P = 1750 N/m² and P = 3050 N/m²) with average sand particle size of 460 μm. Bed temperature distributions across the three risers were measured and compared at different heights (1.04 m, 1.64 m, and 2.24 m above the distributor plate). Axial distribution of heat transfer coefficient along the height of riser was evaluated and compared for the three bed cross sections. Effect of superficial velocity of air, sand inventory, and bed cross section on bed temperature and heat transfer coefficient was investigated. An empirical correlation was developed for the bed Nusselt number as a function of various non-dimensional parameters based on the parametric study. The correlation was compared with available literatures.