Direct contact ultrasound assisted freezing of mushroom (Agaricus bisporus): Growth and size distribution of ice crystals

To reduce the size of ice crystals in mushroom (Agaricus bisporus) contact ultrasound (300 W, 20 kHz) was applied during freezing and frozen storage. Stereomicroscopy was used to observe the ice crystal morphology, and DSC and NMR spectroscopy were performed to evaluate the water states in the samples. Results indicated that ultrasound irradiation initiated the nucleation of ice and reduced the mean size of ice crystals during freezing and frozen storage, and therefore improved the frozen product quality compared to the control samples. Most of the ice crystals in the ultrasound assisted frozen (UAF) samples were in the size range of 0–80 microns while that for the control samples were in the size range of 50–180 microns. SEM photos also proved that due to the application of ultrasound, the sizes of the ice crystals was reduced. This micro-scale information on the documentation of ice crystals will assist in understanding the ice crystal growth phenomena in an ultrasound assisted freezing process.     

Ice nucleation in water droplets on glass surfaces: From micro- to macro-scale

Ice nucleation encountered in engineering systems is often induced by solid/water interfaces. When classical nucleation theory is used to analyze ice nucleation in such systems, the uniformity of interfaces that contribute to ice nucleation must be carefully considered, because classical nucleation theory cannot be directly applied to non-uniform interfaces. In this study, to discuss the uniformity of ice nucleating activity of solid/water interfaces, ice nucleation in water droplets prepared on glass surfaces was investigated for various droplet sizes from micrometer to sub-millimeter. When the interfacial area between water and the glass surface was smaller than 1 × 10⁻¹⁰ m², the ice nucleation temperature showed scatter of about 2 °C, suggesting uniformity of the interface. However, when the interfacial area was larger than 1 × 10⁻⁸ m², the ice nucleation temperature showed large scatter, suggesting the ice nucleating activity was no longer uniform.     

Lithium Azides Induced SnS Quantum Dots for Ultra-Fast and Long-Term Sodium Storage

Tin sulfide (SnS) is an attractive anode for sodium ion batteries (NIBs) because of its high theoretical capacity, while it seriously suffers from the inherently poor conductivity and huge volume variation during the cycling process, leading to inferior lifespan. To intrinsically maximize the sodium storage of SnS, herein, lithium azides (LiN₃)-induced SnS quantum dots (QDs) are first reported using a simple electrospinning strategy, where SnS QDs are uniformly distributed in the carbon fibers. Taking the advantage of LiN₃, which can effectively prevent the growth of crystal nuclei during the thermal treatment, the well-dispersed SnS QDs performs superior Na⁺ transfer kinetics and pseudocapacitive when used as an anode material for NIBs. The 3D SnS quantum dots embedded uniformly in N-doped nanofibers (SnS QDs@NCF) electrodes display superior long cycling life-span (484.6 mAh g⁻¹ after 5800 cycles at 2 A g⁻¹ and 430.9 mAh g⁻¹ after 7880 cycles at 10 A g⁻¹), as well as excellent rate capability (422.3 mAh g⁻¹ at 20 A g⁻¹). This fabrication of transition metal sulfides QDs composites provide a feasible strategy to develop NIBs with long life-span and superior rate capability to pave its practical implementation.     

Pressure drop in round cylindrical headers of parallel flow MCHXs: Pressure loss coefficients for single phase flow

This paper presents the investigation of the pressure drop in headers and development of correlation for pressure loss coefficient for single phase flow through round cylindrical headers of parallel MCHXs. The working fluid was compressed air flowing through header with 1–20 m s⁻¹ based on smallest cross section while the velocity through microchannels was in the range 6–30 m s⁻¹. The experimental results indicate that the pressure loss coefficient of inlet header is a linear function of the ratio of velocities through microchannel tube and header, except for the first two microchannel tubes; the pressure loss coefficient of outlet header is a quadratic function of the ratio of velocities through microchannel tube and header, and decreases as the velocities through upstream microchannel tubes increase. Correlations for predicting pressure drop of inlet header and outlet header are developed and agreement for 98% of experimental data is within a ±15 Pa.     

Characterisation of the hydraulic maldistribution in a heat exchanger by local measurement of convective heat transfer coefficients using infrared thermography

A methodology was developed to characterise the heat exchangers' performance decrease due to two-phase flow maldistribution. It consists in measuring the spatial distribution of the local heat transfer coefficients with a rapid, non-invasive and fluid independent method. The method is based on the infrared (IR) thermography measurement of the temperature response to an oscillating heat flux. The amplitude of the measured temperatures is compared to the solution of an analytical model. The problem is solved iteratively to obtain the heat transfer coefficients. This method has been applied to evaluate the uneven phase distribution of an air–water mixture in a compact heat exchanger. The exchanger is composed of seven multiport flat tubes, a vertical downward header and horizontal channels. Experiments were performed for mass flux from 29 kg m⁻² s⁻¹ to 116 kg m⁻² s⁻¹ and for quality from 0.10 to 0.70.     

R1234ze(E) flow boiling inside a 3.4 mm ID microfin tube

R1234ze(E) has a GWP<1 and a normal boiling temperature approximately 7.3 °C lower than that of R134a; it represents an interesting candidate for its replacement as working fluid in refrigerating machines. The refrigerant charge minimization in refrigerating and air conditioning equipment is a key issue for the new environmental challenges. Mini microfin tubes represent an optimal solution for both heat transfer enhancement and charge minimization tasks. This paper presents an experimental study of R1234ze(E) flow boiling inside a mini microfin tube with internal diameter at the fin tip of 3.4 mm. The experimental measurements were carried out at constant saturation temperature of 30 °C, by varying the refrigerant mass velocity between 190 kg m⁻² s⁻¹ and 940 kg m⁻² s⁻¹, the vapour quality from 0.2 to 0.99 at three different heat fluxes: 10, 25, and 50 kW m⁻². The experimental results are then compared with those obtained for the more traditional R134a.     

Electrochemical reclamation of silver from silver-plating wastewater using static cylinder electrodes and a pulsed electric field

Silver was reclaimed from silver-plating wastewater by using a pulsed electric field (PEF) combined with static cylinder electrodes (SCE). The conditions that produced the maximal silver recovery rate (RR_Ag) (99%) were as follows: average retention time of 10 min, interelectrode gap of 50 mm, solution pH of 9.0, temperature of 45 °C, initial Ag(I) concentration of 1000 mg L⁻¹, PEF pulse frequency of 1200 Hz, current density of 5.0 A m⁻² and a pulse duty cycle of 60%. Compared with the conventional direct current (DC) technology, the PEF process exhibited improvements in the silver recovery rate (RR_Ag), total energy consumption (TEC) and physical properties of the silver deposits, especially for low Ag(I) concentrations, for example, from 500 to 1000 mg L⁻¹. For an initial Ag(I) concentration of 500 mg L⁻¹, the PEF process produced an RR_Ag of up to 99%, and the TEC was 4.56 kWh (kg Ag)⁻¹. In comparison, the RR_Ag and TEC were 90% and 5.66 kWh (kg Ag)⁻¹, respectively, in the DC process. The results of SEM observation and XRD analysis indicated that the silver deposits formed by the PEF process were smaller, denser, and of a higher purity than those produced by the DC process. Therefore, the presented method was effective for reclaiming silver from silver-plating wastewater.     

Experimental study on thermal characteristics of ammonia flow boiling in a plate evaporator at low mass flux

Thermal characteristics of a plate evaporator using ammonia are experimentally investigated. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient of the evaporator are discussed. The experiments are conducted for mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is obtained as a function of quality for all experimental conditions. The characteristics of heat transfer coefficient are discussed and compared with those of earlier works. All experimental results are compiled by using Lockhart–Martinelli parameter. The developed empirical correlation predicts 85% of the experimental data within ±30% range.     

Experimental study on cooling performance of air conditioning system with dual independent evaporative condenser

Evaporative condenser is an energy efficient and environmentally friendly air conditioning equipment. This paper proposed an air conditioning system using dual independent evaporative condenser and investigated the cooling performance. Many factors, such as evaporator water inlet temperature, compressor frequency, air dry-bulb temperature, air velocity and water spray rate, which influenced the cooling performances of air conditioning system with evaporative condenser have been investigated. The results indicated that cooling capacity and coefficient of performance (COP) increased significantly with the increasing of evaporator water inlet temperature (12–25 °C), the air velocity (2.05–3.97 m s⁻¹) and the water spray rate (0.03–0.05 kg m⁻¹ s). However, COP decreased with the increasing ambient air dry-bulb temperature (31.2–35.1 °C) and the compressor frequency (50–90 Hz). Furthermore, the heat transfer coefficient (K₀) was 232–409 W m⁻² K⁻¹ in different air velocity and water spray rate.     

Surface tension of low GWP refrigerants R1243zf,R1234ze(Z), and R1233zd(E)

The surface tension of R1243zf, R1234ze(Z), and R1233zd(E) were measured at temperatures from 270 K to 360 K by an experimental apparatus based on the differential capillary rise method. The deviation between the measured surface tension of R134a and R245fa and the calculated surface tension with REFPROP 9.1 (Lemmon et al., 2013) was ±0.13 mN m⁻¹, which is less than the estimated propagated uncertainty in surface tension of ±0.2 mN m⁻¹. Eleven points, thirteen points, and ten points of surface tension data were provided for R1243zf, R1234ze(Z), and R1233zd(E), respectively, in this paper. The measured data and the estimated surface tension using the methods of Miller, 1963, Miqueu et al., 2000, and Di Nicola et al. (2011) agree within the standard deviation of ±0.43 mN m⁻¹. The empirical correlations that represent the measured data within ±0.14 mN m⁻¹ were proposed for each refrigerant.     

Nucleate boiling heat transfer coefficients of HFO1234yf on various enhanced surfaces

In this study, nucleate boiling heat transfer coefficients (HTCs) of HFO1234yf HFC134a are measured on a flat plain, Turbo-B, Turbo-C, and Thermoexcel-E surfaces. All data are taken at the liquid pool temperature of 7 °C on small flat horizontal square copper plates (9.53 mm × 9.53 mm) at heat fluxes from 10 kW m⁻² to 200 kW m⁻² with an interval of 10 kW m⁻². Test results show that nucleate boiling HTCs of HFO1234yf on all four surfaces are similar to those of HFC134a at all heat fluxes tested in this study. At heat fluxes below 150 kW m⁻², Thermoexcel-E surface shows the highest heat transfer performance and hence is the best surface for the manufacture of the evaporators in refrigeration and air-conditioning equipment. On the other hand, at high heat fluxes above 150 kW m⁻², Turbo-B and Turbo-C show better heat transfer performance than Thermoexcel-E and hence are good for electronic cooling applications. Overall, HFO1234yf is a good long term candidate with excellent environmental properties to replace successfully HFC134a from the view point of pool boiling heat transfer. Hence HFO1234yf can be readily applied to the conventional evaporators designed for HFC134a.     

Flow boiling data and prediction method for enhanced boiling tubes and tube bundles with R-134a and R-236fa including a comparison with falling film evaporation

New enhanced boiling tubes from Wolverine Tube Inc. (Turbo-B5) and Wieland-Werke AG (Gewa-B5) were investigated using R-134a and R-236fa as test fluids. The tests were done at saturation temperatures of 5 and 15 °C, mass flow rates from 4 to 35 kg m⁻² s⁻¹ and heat fluxes from 15 to 70 kW m⁻². A new prediction method based on a theoretical analysis of thin film evaporation was used to propose a new correlating parameter. A large new database of local heat transfer coefficients was obtained and utilized to generate an improved prediction method for bundle boiling and the onset of dryout. Onset of dryout and the simultaneous reduction in heat transfer performance occurred at very high vapour quality on these enhanced tubes in convective bundle boiling. Furthermore, a direct comparison was made between the tubes operating in falling film and convective bundle boiling modes.     

Effects of different freezing methods on the quality and microstructure of lotus (Nelumbo nucifera) root

The effects of three freezing methods, air blast freezing (ABF), immersion freezing (IF) and ultrasound-assisted immersion freezing (UIF), on quality and microstructure of lotus roots were investigated. The parameters used to evaluate the freezing methods effect were the freezing time, color, firmness, drip loss, vitamin C and microstructure of the final frozen products. The results showed that the UIF products had several advantages in terms of the freezing time, color, firmness and drip loss over ABF and IF. No significant difference (p > 0.05) of vitamin C content was observed between the ABF and IF products, while significant difference (p < 0.05) of vitamin C was observed between UIF and ABF/IF products. ABF caused the largest destruction to the tissue, while the microstructure of the UIF products was the best preserved. It is concluded that UIF processing was a better freezing method for lotus root with improved quality and less damaged microstructure than the two other methods.     

Three-dimensional measurement of ice crystals in frozen dilute solution

A Micro-Slicer Image Processing System (MSIPS) has been applied to observe the ice crystal structures formed in frozen dilute solutions. Several characteristic parameters were also proposed to investigate the three-dimensional (3-D) morphology and distribution of ice crystals, based on their reconstructed images obtained by multi-slicing a frozen sample with the thickness of 5 μm. The values of characteristic parameters were determined for the sample images with the dimension of 530×700×1000 μm. The 3-D morphology of ice crystals was found to be a bundle of continuous or dendrite columns at any freezing condition. The equivalent diameter of ice crystals were in the range of 73–169 μm, and decreased exponentially with increasing freezing rate at the copper cooling plate temperature of −20 to −80 °C. At the T_cp −40 °C, the volumes of ice crystals were in the range of 4.6×10⁴ μm³ to 3.3×10⁷ μm³, and 36 ice columns were counted in the 3-D image.     

Early age electrical resistivity behaviour of various concrete mixtures subject to low temperature cycling

The effect of thermal cycling between −24 and + 24 °C on the electrical resistivity of five concrete mixtures was investigated at early-ages. Each mixture was subjected to two thermal cycles (initiated either 1 or 14 days after casting) wherein temperatures were changed by 1 °C/h. Electrical resistivity increased as temperature decreased. This relationship followed the Arrhenius equation until the phase transition temperature (i.e. initial freezing of the pore solution), at which point electrical resistivity increased greatly. A method of determining the phase transition temperature is presented that could be used in place of existing methods using mortar cubes. The phase transition temperature was higher during thawing cycles compared to freezing cycles for all mixtures. Mixtures with ground slag replacing cement had higher resistivity and lower phase transition temperatures than mixtures using only cement due to the change in the ionic combination of the concrete pore solution. The phase transition temperature decreases as concrete ages.     

A modified model of solar collector/regenerator considering effect of the glazing temperature

Solar liquid collector/regenerator (C/R), combining the functions of solar collector and regenerator of absorbent solution together, can be effectively utilized in solar energy-driven liquid desiccant cooling systems. Based on thermal balance of the glazing of solar C/R, a group of modified heat and mass transfer models, validated by experimental results to reflect solution regeneration process more truly, were put forward in this paper. Numerical simulation showed only preheating air stream, keeping an equal humidity ratio, did raise the performance of solar C/R, but preheating solution increased the regeneration efficiencies to reach twice that of preheating air stream. There occurred optimum mass flow rates for both air stream and solution film reaching 36–48 kg m⁻¹ h⁻¹ and 4∼6 kg m⁻¹ h⁻¹ respectively for solar C/Rs of 3∼6 m long. As for effect of the length of solar C/Rs, the regeneration efficiency η_r reached a maximum value at about 4 m and shorter or longer solar C/Rs failed to increase solution regeneration efficiencies.     

Evaporation heat transfer and pressure drop of ammonia in a mixed configuration chevron plate heat exchanger

Ammonia is a naturally occurring environment friendly refrigerant with attractive thermo-physical properties. Experimental investigation of heat transfer and pressure drop during steady state evaporation of ammonia in a commercial plate heat exchanger has been carried out for an un-symmetric 30°/60° chevron plate configuration. Experiments were conducted for saturation temperatures ranging from −25 °C to −2 °C. The heat flux was varied between 21 kW m⁻² and 44 kW m⁻². Experimental results show significant effect of saturation temperature, heat flux and exit vapor quality on heat transfer coefficient and pressure drop. Current mixed plate configuration data are compared with previous studies on the same heat exchanger with symmetric plate configurations. This comparison highlighted importance of optimization in selection of the heat exchangers. Correlations for two phase Nusselt number and friction factor for each chevron plate configuration considered are developed. A Nusselt number correlation generalized for a range of chevron angles is also proposed.     

Measurement and prediction of heat transfer coefficient on ammonia flow boiling in a microfin plate evaporator

Thermal characteristics of ammonia flow boiling in a microfin plate evaporator are experimentally investigated. Titanium microfin heat transfer surface is manufactured to enhance boiling heat transfer. Longitudinally- and laterally-microfined surfaces are used and those performances are compared. Heat transfer coefficient of microfin plate evaporator is also compared with that of plain-surface plate evaporator. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient are presented and discussed. The experiments are conducted for the range of mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is compared with that predicted by available empirical correlations proposed by other researchers. Modified correlations using Lockhart-Martinelli parameter to predict heat transfer coefficient are developed and they cover more than 87% of the experimental data.     

Condensation heat transfer and pressure drop in flattened microfin tubes having different aspect ratios

In this study, condensation heat transfer coefficients and pressure drops of R-410A are obtained in flattened microfin tubes made from 7.0 mm O.D. round microfin tubes. The test range covers saturation temperature 45 °C, mass flux 100–400 kg m⁻² s⁻¹ and quality 0.2–0.8. Results show that the effect of aspect ratio on condensation heat transfer coefficient is dependent on the flow pattern. For annular flow, the heat transfer coefficient increases as aspect ratio increases. For stratified flow, however, the heat transfer coefficient decreases as aspect ratio increases. The pressure drop always increases as aspect ratio increases. Possible reasoning is provided based on the estimated flow pattern in flat microfin tubes. Comparison with existing round microfin tube correlations is made.     

The magnetocaloric effect in (Gd0.74Tb0.26)5(SixGe1−x)4 alloys

The magnetic phase transition and magnetocaloric effect of the alloys of Gd_0.74Tb_0.26 and (Gd_0.74Tb_0.26)₅(Si_xGe_1−x)₄ (x = 0.43, 0.50, 0.505) have been investigated by magnetization measurement. Experimental results show that partial substitution of the Gd by Tb in Gd₅(Si_xGe_1−x)₄ system keeps the first order magnetic transition. Although the values of transition temperature decrease, the as-cast (Gd_0.74Tb_0.26)₅(Si_0.43Ge_0.57)₄ and annealed (Gd_0.74Tb_0.26)₅(Si_0.50Ge_0.50)₄ alloys display large magnetic entropy change up to 18.89 J kg^− 1 K^− 1 and 13.79 J kg^− 1 K^− 1 near their transition temperatures in the low magnetic field change of 0–2.0 T, respectively.