Modeling the thawing of frozen foods using air impingement technology

With the continual growth in the use of frozen foods both in retail and in food service, there is a need to develop improved thawing methods. Current methods are often undesirably slow (still air) or are very expensive and cause uneven thawing (microwave). Air impingement technology is one possible method to improve the thawing of frozen foods. The objectives of this research were to develop a two-dimensional model for air impingement thawing frozen foods and to verify the model experimentally. Frozen products were thawed using a laboratory impingement system with a single impingement jet. A simulated meat product (Tylose gel) was used as the test material. Thawing of a Tylose disk (12.7 cm diameter, 1.98 cm thickness) with air at 6 °C without impingement required more than 12 h, while thawing under a single impingement jet took less than 3 h, over four times faster. Results from the finite difference model gave good agreement with experimental data. Moisture loss during thawing was typically over-predicted because moisture gain due to condensation was not modeled.     

Jet entrainment rate in air curtain of open refrigerated display cases

Experimental and computational methods are used to address those parameters that have significant effects on the amount of entrained (warm) air in an open refrigerated display case. These parameters are identified, quantified, and the rate of entrainment is expressed as a function of these parameters. It is found that the turbulence intensity, the shape of the mean velocity profile at the discharge air grill, and the Reynolds number are mainly responsible for the amount of entrained air in a display case. It is also concluded that lower Reynolds numbers will reduce the amount of the entrained air in the display case, however, the trade off will be higher temperature of the food products on shelves. Digital particle image velocimetry (DPIV) was used to map the entire mean velocity flowfield and the turbulence intensity. The laser doppler velocimetry technique was also used to verify the mean velocity and turbulence intensity measurements made by DPIV. The results indicated an excellent agreement between both methods. Parametric studies for the rate of entrainment of the outside air into the display case were performed using a computational fluid dynamics tool. The results indicate that lowering the Reynolds number of the air curtain reduces the entrainment rate. However, sufficiently high momentum should still exist to enforce the integrity of the air curtain structure.     

Experimental performance analysis and modelling of liquid desiccant cooling systems for air conditioning in residential buildings

The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7 g kg⁻¹ dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20 °C for the summer design conditions of 32 °C, 40% relative humidity. With air volume flow rates of 200 m³ h⁻¹ the system can provide 886 W of cooling power.A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved.     

Thermal uniformity and rapid cooling inside refrigerators

The authors developed a new air-supply system for improving the thermal uniformity and the cooling rate inside a fresh food cabinet of a household refrigerator. For these purposes, we added a blower and jet slots to a conventional cooled air supply system. The jet slots circulate the air inside the cabinet at a higher velocity to optimize airflow velocity and its distribution. The jet stirs the air inside the cabinet and improves thermal uniformity, which resulted in half the temperature deviation in the cabinet as that of the conventional systems. The jet also improves the heat transfer on the surface of foods. We achieved a four times higher cooling rate with the new system than that with the conventional ones. In order to cut down the development period, we applied computational fluid dynamics to study air distribution inside the cabinet with the new system. We also derived the model of the cooling process by the jet using theoretical and empirical equations and applied it to decide the jet velocity for the rapid cooling.     

Freezing time and rate of cauliflower florets under different freezing conditions

In this study, the freezing time and rate of 1 cm³ cauliflower floret samples were determined under different freezing conditions in an air blast freezer. Four different air temperatures (−20, −25, −30 and −35°C) and six different air velocities (70, 131, 189, 244, 280 and 293 m min⁻¹) were applied in the freezer, and the freezing rate and time of cauliflower pieces were determined under each condition. The freezing time of cauliflower samples frozen with cold air at −20°C and 280 m min⁻¹ was similar to that of samples frozen with cold air at −35°C and 70 m min⁻¹. When the velocity of air was increased from 70 m min⁻¹ to 293 m min⁻¹, the freezing time was approximately halved.     

The impact of fouling on the performance of filter–evaporator combinations

This paper presents results of experiments performed on different combinations of five types of filters of varying efficiencies (MERV4, 6, 8, 11, and 14) and four types of evaporator coils with depths and fin geometries under clean and fouled conditions. The fouled conditions were obtained after injection of 600 g (1612 g/m² of coil face area) of dust upstream of the filter–coil combination, which was meant to simulate a year of operation in the field. The air-side pressure drops of the coils and filters and air-side effective heat transfer coefficients of the coils were determined from the measurements under the clean and fouled conditions. Depending upon the filter and coil, the coil pressure drops increased in the range of 6–30% for an air velocity of 2.54 m/s. The impact was significantly greater for tests performed without an upstream filter (the coil pressure drops increased from 43% to 200%). The largest relative effect of fouling on pressure drop occurs for coils with fewer rows, primarily due to higher fin densities. The impact of fouling on air-side effective heat transfer coefficients was found to be relatively small, which ranged from −14% to 4%. In some cases, heat transfer was actually enhanced due to additional turbulence caused by the presence of dust. However, with large dust deposits, heat transfer is degraded because the dust also acts as insulation and creates an uneven air velocity.     

Axial temperature distribution in vertical jet fires

The behaviour of vertical commercial propane jet fires (flame lengths of up to 8 m) was studied experimentally. The temperatures along the jet fire centreline were measured using a set of thermocouples and the flame contour was determined from infra-red (IR) images. The results show that temperature increases from the bottom of the flame, reaches a maximum value and decreases again at the top zone. A second-degree polynomial expression describes fairly well the variation of temperature as a function of the position on the flame centreline. The temperature along the centreline was found to increase for Q values lower than 7 MW and to decrease at higher values.     

Improvement of air distribution in refrigerated vertical open front remote supermarket display cases

This paper presents some of the results derived from extensive experimentation on display cases for supermarkets and derives some possible improvements to augment temperature uniformity and energy performance. The effect of the perforation pattern of the rear duct on the distribution of airflow between these and the front curtain is brought out. The critical component is air infiltration across the curtain, which is governed by internal air distribution and curtain characteristics. It is observed that a 70–30 distribution of flow between the curtain and the rear duct perforations yields a performance that satisfies the standards. Further, a judicious distribution of perforations on the rear duct at various levels and across is necessary. Correlations available in the literature are useful in making a qualitative assessment of the results.     

Development of a circulating system for a jet refrigeration cycle

This paper proposed a workless-generator-feeding (WGF) system for a jet refrigeration cycle, using R141b. This feeding system does not require any mechanical power. The liquid refrigerant from the condenser was fed to the vapour-generator by means of the generator pressure and gravitational force. The system was tested and compared with a conventional system using a mechanical pump. It was found that this system was workable. The heat input to the generator was slightly higher than that for a system using a mechanical pump. The jet refrigeration cycle employing this new feeding system provided a slightly lower coefficient of performance (COP) compared to a system using a mechanical pump. However, this new system did not require any mechanical energy. Therefore, the jet refrigeration system employing this WGF system is truly a heat-power refrigeration cycle.     

A theoretical and experimental study of a small-scale steam jet refrigerator

The paper provides the results of a theoretical and experimental study of a steam jet refrigerator. A small-capacity steam jet refrigerator has been tested with boiler temperatures in the range 120–140°C. The experimental data were found to be within 85% of the theoretical values. The experiments showed that choking of the secondary flow in the mixing chamber of the ejector plays an important role in the system performance. Maximum COP was obtained when the ejector was operated at its critical flow condition. Off-design performance characteristics of the system are provided.     

Theoretical and experimental freezing times of strawberries

Strawberries were frozen at different air velocities in an air blast freezer at −30°C. The freezing time was taken as the time required to lower the temperature at the geometric centre of the samples to −10°C. The freezing times measured in the experiments were compared with the values calculated using Plank's equation. The freezing times calculated by Plank's equation were fround to be higher than those found experimentally at any given air velocity. Freezing time decreased with increasing air velocity. This is attributed to the increase in heat transfer coefficient at increased air velocities.     

Frost formation on a cold surface under turbulent flow

This study presents a mathematical model to predict the frosting behavior on a cold surface under turbulent flow. The model consists of the standard κ–ε model for turbulent flow and the diffusion equation for the frost layer. The numerical results show that turbulent flow promotes the growth of the frost layer on the cold surface, compared to the laminar flow. Increase in air velocity has little effect on mass transfer under turbulent flow, while frost growth under laminar flow is influenced by the air velocity. With constant air humidity, the frost layer thickness increases with decreasing air temperature, while the relationship for the frost density is reversed. The effect of the air temperature on the mass flux is negligible, compared to the other frosting parameters.     

Two- and three-dimensional CFD applied to vertical display cabinets simulation

A commercial CFD code has been employed to simulate the air flow pattern and the temperature distribution in a frozen food vertical display cabinet. At first the choice of solver parameters has been investigated in a 2D modelisation. 3D simulations have been then performed, and the effects of the cabinet length, of the warm air curtain and of longitudinal ambient air movement have been investigated. The results show that, in short cabinets, 3D secondary vortices at the side walls provide the most important mechanism for hot air entrainment. Comparison with experimental results shows that a 2D simulation is totally inadequate for such configurations, while 3D computations predict refrigeration power within engineering accuracy. Furthermore, the computed refrigerating power shows that even low room air velocity, due to its interaction with the end-wall vortices, has a significant impact on cabinet performance.     

The effect of radiation shields around the air condenser and compressor of a refrigerator on the temperature distribution inside it

In almost all domestic refrigerators–freezers all components are assembled in the same relative position since several years ago. It is also known that the condenser releases heat at high temperatures (first law of thermodynamics) as well as the compressor. This heat is rejected to the environment in almost all practical situations partially by natural air convection. However, part of it is due to thermal radiation that causes an overheating of the refrigerator–freezer surfaces adjacent to those equipments. As a consequence there are more heat gains to the refrigerator–freezer through these surfaces and hence higher air temperatures inside. This paper describes how a simple technique can be very useful in order to minimize that part of heat transfer by radiation. The improvement is achieved by placing a radiation shield – a sheet of aluminium foil – over the surfaces close to the condenser and the compressor. For validating this technique a refrigerator–freezer was monitored with thermocouples for the measurements of the inside air temperatures in two situations: with and without the radiation shield. Results show that with this practice the average inside air temperatures in the refrigerator–freezer could decrease to about 2 K. An available commercial code was used in order to simulate the air temperature distribution and air velocities inside the refrigerator cabinet in both situations. Results from the experimental apparatus and from simulations show that there is a good agreement between them which validates the experiments carried out.Also an available commercial code, the Fluent, was used to simulate the internal air temperature in both situations.     

Study of the jet flow field of vacuum spray process

In this work, we study the velocity and turbulence distribution of the cone nozzle jet flow field into a vacuum chamber under different nozzle diameters and injection pressures. The simulations are carried out with using the software based on the volume of fluid method, the standard k-ε turbulence model and pressure implicit with splitting of operator arithmetic. It is found that for injection pressure lower than 10 MPa, the spray penetration increases with the increase of injection pressure. However, for injection pressure higher than 10 MPa, the spray penetration tends to reduce progressively. Moreover, the distribution of velocity and turbulence become worse than that of the spray jets with injection pressure lower than 10 MPa. The potential core, spray penetration and diffusion zone of the spray jet increase with the increase of the nozzle diameter. Furthermore, without considering the solubility of the target polymer materials, acetone solvent shows better jet flow distribution in comparison with that of chloroform and carbon tetrachloride.     

Airflow blockage and guide technology on energy saving for spiral quick-freezer

The effect of airflow blockage and guide technology on energy saving for spiral quick-freezers were investigated by simulating and analyzing the airflow field and measuring of the velocity distribution in the freezing zone for different designs. The k– turbulence model was used. The velocities and temperatures of the air in the freezing zone for different designs of airflow blockage and guide boards were measured. The study shows that the airflow pattern plays a key role on energy efficiency, freezing time, and production rate. In the study case, through the optimization of the airflow blocking boards and the guide boards, the average air velocity in the freezing zone would be enhanced to 2.5–2.7 times compared with the original design. Correspondingly for bean curds in a stationary condition, the freezing time would be shortened by 78–85%, energy efficiency and the production rate would be increased by approximately 18–28% individually.     

A theoretical study of a novel regenerative ejector refrigeration cycle

There has been a demand for developments of the ejector refrigeration systems using low grade thermal energy, such as solar energy and waste heat. In this paper, a novel regenerative ejector refrigeration cycle was described, which uses an auxiliary jet pump and a conventional regenerator to enhance the performance of the novel cycle. The theoretical analysis on the performance characteristics was carried out for the novel cycle with the refrigerant R141b. Compared with the conventional cycle, the simulation results show that the coefficient of performance (COP) of the novel cycle increases, respectively, by from 9.3 to 12.1% when generating temperature is in a range of 80–160 °C, the condensing temperature is in a range of 35–45 °C and the evaporating temperature is fixed at 10 °C. Especially due to the enhanced regeneration with increasing the pump outlet pressure, the improvement of COP of the novel cycle is approached to 17.8% compared with that in the conventional cycle under the operating condition that generating temperature is 100 °C, condensing temperature is 40 °C and evaporating temperature is 10 °C. Therefore, the characteristics of the novel cycle performance show its promise in using low grade thermal energy for the ejector refrigeration system.     

卧式陈列柜风幕的数值分析

利用CFD软件phoenics,通过对卧式陈列柜传热过程的分析,分别在忽略辐射和不忽略辐射2种情况下建立陈列柜风幕的k-ε数学模型,模拟计算不同送风速度下2种模型的风幕性能以及柜体热负荷,并与试验数据进行对比.结果表明,考虑辐射的模型的模拟结果更符合试验数据.同时对比不同送风速度对风幕性能的影响,风幕送风速度过小,冷射流不能到达回风口,不能形成完整的风幕;风幕送风速度的过大,增加了风幕射流与环境热空气的热质交换,使得柜内温度升高,所以必须选择合理的送风速度.     

Heat and mass transfer characteristics in a spray chamber

This paper presents a one-dimensional mathematical model for heat and mass transfer of water droplets in a spray chamber. The model includes drop size distribution and velocity of the droplets generated by a nozzle of inlet diameter 3.2 mm. By using the conservation of mass and energy, the changes in water temperature, air temperature and humidity along the spray cone in the spray chamber can be calculated. This model is tested with two different water mass flows. The results look reasonable from practical point of view and they also show that higher water mass flow results in a higher air temperature drop and higher humidity.