Experimental strategies for frost analysis (original) (raw)
Related papers
Principles of Frost Protection
More economic losses occur due to freeze damage in the United States than to any other weather related hazard. Consequently, considerable effort to reduce damage is expended. The cost effectiveness depends on the frequency of occurrence, cost of the production method, and the value of the crop. Generally, passive freeze protection is easily justified. Passive protection includes practices done before a freeze night that reduces the potential for damage. Active protection includes energy intensive practices (heaters, sprinklers, wind machines, etc.) that are used during the freeze night to replace natural energy losses. Active protection is sometimes not cost effective. In this lecture, forecast freezing temperatures and passive protection will be discussed. TYPES OF FROST EVENTS Advection Frost An advection frost occurs when cold air blows into an area to replace warmer air that was present before the weather change. It is associated with moderate to strong winds, no temperature inversion, and low humidity. Often temperatures will drop below 32°F (0°F) and stay there all day. Advection frosts are difficult to protect against, but fortunately they are rare in California fruit growing regions. occurs when the temperatures aloft are only slightly higher than near the surface. When there is a strong inversion (low ceiling), temperature increases rapidly with height. Most frost protection methods are more effective during low ceiling, strong inversion conditions. ENERGY TRANSFER Energy or heat transfer determines how cold it will get and the effectiveness of protection. The four methods of energy transfer are radiation, conduction, convection, and latent heat. Understanding these heat transfer mechanisms is extremely important for good frost protection management. Radiation Radiation is electromagnetic energy transfer. A good example of radiation is sunlight. Because it is very hot, considerable energy is radiated from the sun to Earth. Although much cooler, objects on Earth also radiate energy to their surroundings. If an object radiates more energy than it receives from other sources, it will cool. Conduction Conduction is heat transfer through matter where the objects do not move. A good example is the transfer of heat through a metal rod if one end is placed in a fire. The heat is transferred by conduction to the other end of the rod. Conduction is important in soil heat transfer and hence frost protection. Convection Convection is the process where a fluid (e.g. air or water) is heated and physically moves from one place to another and takes heat with it. Air heated by smudge pots is an example of convection because the air, warmed by the heaters, rises and mixes with colder air in the orchard to raise the temperature. Smudge pots also radiate heat to nearby trees but the main protection comes from convection. Latent Heat When water condenses, cools, or freezes, the temperature of the environment around the water rises because latent is changed to sensible heat. Latent heat is chemical energy stored in the bonds that join water molecules together and sensible heat is heat you measure with a thermometer. When latent heat is changed to sensible heat, the air temperature rises. When ice melts, water warms, or water evaporates, sensible heat is changed to latent heat and the air temperature falls. Table 1 shows the amount of heat consumed or released per unit mass for each of the processes. Process Heat Exchange Calories per gram Water cools from 20 o C (68°F) to 0 o C (32°F) +20.0 Water freezes at 0°C (32°F) +79.7 Ice cools from 0°C (32°F) to-5 °C (23°F) +2.5 Water evaporates at 0 °C (32 °F)-597.3 Water condenses at 0 °C (32 °F) +597.3 Water sublimates (ice to water vapor) at 0 °C (32 °F)-677.0 Water deposits (water vapor to ice) at 0 °C (32 °F) +677.0
Energy Engineering
The frost growth on cold surfaces in evaporators is an undesirable phenomenon which becomes a problem for the thermal efficiency of the refrigeration systems because the ice layer acts as a thermal insulation, drastically reducing the rate of heat transfer in the system. Its accumulation implies an increase in energy demand and a decrease in the performance of various components involved in the refrigeration process, reducing its efficiency and making it necessary to periodically remove the frost, resulting in expenses for the defrost process. In the present work, a numerical-experimental analysis was performed in order to understand the formation process of porous ice in flat plates with different surface treatments and parameters. This understanding is of utmost importance to minimize the formation of porous ice on cold surfaces and improve equipment efficiency and performance. In this context, a low-cost experimental apparatus was developed, enabling an experimental analysis of the phenomenon under study. The environmental conditions evaluated are the temperature of the cold surface, room temperature, humidity, and air velocity. The material of the surfaces under study are aluminum, copper, and brass with different surface finishes, designated as smooth, grooved (hydrophilic), and varnished (hydrophobic). The numerical-experimental analysis demonstrates measurements and simulations of the thickness, surface temperature, and growth rate of the porous ice layer as a function of the elapsed time. The numerical results were in good agreement with the experimental results, indicating that the varnished surface, with hydrophobic characteristics, presents greater difficulty in providing the phenomenon. Therefore, the results showed that application of a coating allowed a significant reduction on the frost formation process contributing to the improvement of thermal efficiency and performance of refrigeration systems.
Current status and future trends of computational methods to predict frost formation
2018
Nowadays, the increasing energy prices and associated environmental concerns lead the refrigeration systems' developers and manufacturers to develop more energy efficient and sustainable equipment and devices. On the most demanding systems, intense usage results in the fast accumulation of ice on the evaporator fins that reduces the efficiency and may even clog the system. These systems often have time-controlled defrost cycles, that heat the evaporator, melting the ice and allowing the system to keep working normally after the defrost cycle. This cycle consumes extra energy and causes a thermal imbalance on the refrigerated space, that may result in a worst refrigeration quality. If it was possible to avoid the defrosting cycle passively (without energy consumption) its efficiency would greatly increase, and the refrigeration temperature would be more stable. Currently defrost cycles cannot be avoided in an economically viable way, although new designs, materials and configurations show promising results, and are currently being investigated. These studies require experimental tests that may become expensive as several geometries, topologies, materials and surface treatment combinations should be evaluated. To access the efficiency before these experimental tests, computational models that simulate frost formation could predict with some accuracy which of the most promising configurations should be then tested experimentally. The present paper aims to review the computational methods to predict frost formation and compare them for possible usage in the computational study of evaporators. Additionally, the future trends of the simulations are discussed, taking into account physical and mathematical models, numerical procedures and the accuracy of the dynamic pattern of the predictions.
Correct simulation of real frost attack in laboratory tests
ConcreteLife'06-International RILEM-JCI Seminar on Concrete Durability and Service Life Planning: Curing, Crack Control, Performance in Harsh Environments, 2006
In another contribution to this symposium [1] the author showed that frost suction is the most important phenomenon in any freeze-thaw attack. It precedes the damages due to frost action. It is far more efficient than any other transport e.g. like isothermal capillary suction. It is linked to the transient and combined heat and liquid transport during a freeze-thaw cycle. The boundary conditions are extremely important. When reproducing real conditions in laboratory testing the boundary conditions of real attack must be simulated with care. In the RILEM recommendations of CDF and CIF test this prerequisite is consequently fulfilled for the first time. Due to this they are efficient and precise procedures approved following ISO 5725. These tests allow simultaneously the determination of capillary and frost suction, the internal damage and the scaling. Scaling dominates in the combined attack of frost and deicing chemicals, internal damage in pure frost attack. The frost suction could be used to measure the transport of other detrimental substances dissolved in water and transported with it e.g. of chlorides.
There is every possibility of formation of ice on the surface of the cooling coil, if the air is used as source of heat either in primary circuit or in secondary circuit. This is because the moisture in the air will come out in the form of dew when the air is cooled in the cooling coil below both water freezing temperature and air dew point temperature, the frost will start to form. In other words, the frost starts to form when the contact happens between the cold surface of the heat exchanger (HX) and the near water vapor in the air due to the temperature difference. The colder the evaporator, the more water vapor will freeze out of the air. Hence the evaporator temperature must be as high as possible, while maintaining the desired room or fixture temperature. This means keeping a minimum or low-temperature difference (TD) between the entering air and the evaporator. A low TD is about 8 to 10°C. Four conditions help achieve a low TD: 1] a large evaporator surface, 2] a fully active evaporator, 3] a rapid circulation of air, 4] a clean, frost-free surface. These conditions for a given temperature promote slow frost buildup, high-suction pressure, high capacity, and efficiency. Less water vapor is removed from the air as frost. Humidity remains high. Foods lose less moisture and weight, and they keep their best appearance. Yet the Relative humidity (RH) also plays a major effect on the frost formation. Normally the frost growth rate is comparatively slow when RH is less than 40%. But when RH is high with high-temperature difference amid the cold surface and the surrounding air, the growth rate will appreciably increase. Initially, the influence of frost on the performance is negligible, but later the frost will start to accumulate more and more and reduce the micro-gap between fins and tubes. As a result, this will affect the whole system due to the partially airflow blockage or full blockage. Yet the contact resistance will reduce the heat transfer rate although the air-side heat transfer coefficient may increase moderately at the beginning stage. Notice that a further accumulation of the frost layers may jeopardize heat transfer performance and cause a much higher air-side pressure drop (ΔP) and increase the power consumption at the same time, and a typical power increase is in association with fan operation. Consequently, the coefficient of performance (COP) or the capacity of the refrigeration system will degrade appreciably and sometimes may even lead to system shutdown. The fan blades are also likely to be damaged if ice builds up on the fan ring. Frosting is unavoidable either in domestic refrigerator or in industrial installation.
Energy Conversion and Management, 2009
In this study, a direct new formula that predicts frost formation on cold walls corresponding to psychrometric-subsaturated. The new formula uses data of the entering air dry-bulb temperature and absolute humidity and absolute humidity of air at saturation corresponding to the coil surface temperature. To validate the formula, case studies of demarcation criteria for frost formation on evaporator coil using experimental measured data and on walls of cold storage freezer using measured data from literature are used. Results completely match with the graphic plot of the data on the psychrometric chart. In case of cold storage freezers, results clearly show that a greater demarcation criteria value indicates frost formation under severe condition, as snow-like with low density and thermal conductivity.
Evaporator Frosting in Refrigerating Appliances: Fundamentals and Applications
Energies
Modern refrigerators are equipped with fan-supplied evaporators often tailor-made to mitigate the impacts of frost accretion, not only in terms of frost blocking, which depletes the cooling capacity and therefore the refrigerator coefficient of performance (COP), but also to allow optimal defrosting, thereby avoiding the undesired consequences of condensate retention and additional thermal loads. Evaporator design for frosting conditions can be done either empirically through trial-and-error approaches or using simulation models suitable to predict the distribution of the frost mass along the finned coil. Albeit the former is mandatory for robustness verification prior to product approval, it has been advocated that the latter speeds up the design process and reduces the costs of the engineering undertaking. Therefore, this article is aimed at summarizing the required foundations for the design of efficient evaporators and defrosting systems with minimized performance impacts due to...