1Direct contact heat transfer technology and its application in cold storage

11 basic principles and characteristics of direct contact heat transfer

(a) and (b) are schematic diagrams of conventional coil-type heat exchange and direct contact heat exchange applied to a cold storage system, respectively. It can be seen from these two heat exchange methods that the biggest difference between the direct contact heat exchange and the conventional heat exchange method is: (1) From the structural point of view, it eliminates the evaporation coil in the heat exchanger and makes the structure More simplified; (2) From the working mode, the two media of the coiled heat exchanger are separate (ie, do not touch each other), as in the illustrated system, the refrigerant evaporates in the coil, and the water is outside the coil. ice. The two processes of the direct contact system meet in the heat exchanger. The refrigerant is directly sprayed into the water and vaporized in the water. The water forms ice crystals in the process of contact with the refrigerant. In the upper part of the heat exchanger, the vapor phase refrigeration The agent is separated from the water.

Therefore, direct contact heat exchange has the following advantages over other heat exchanges.

(1) Due to the direct contact between the two media, the heat transfer process between the two media does not have the heat transfer resistance between the liquid D solid and the D liquid, and there is no dirt thermal resistance caused by the fouling of the medium on the pipe wall. Can greatly improve heat transfer efficiency.

(2) Since the coil occupies a considerable share of the material consumption of the system, the direct contact heat exchange can save the coil of the system, which not only makes the system simple, but also saves materials and reduces manufacturing costs.

(3) The volume of the whole set of equipment is greatly reduced.

(4) Since the coil is omitted and the scale inside the coil is avoided, it is expected to reduce the maintenance workload of the equipment.

It can be seen from the above characteristics that direct contact heat exchange has broad application prospects.

Research Status of Application of Direct Contact Heat Exchange in Cool Storage

Direct contact heat transfer exists in many natural phenomena and engineering problems, such as seawater desalination, geothermal recovery, ocean energy conversion, and energy storage. The direct contact heat exchange process in the cold storage includes evaporation of the liquid refrigerant and cooling or solidification of the water after the direct contact with the refrigerant to form two phase-changing thermal processes of ice crystals (or gas hydrates).

Since direct contact heat transfer has high heat transfer efficiency, direct contact heat transfer technology has been widely concerned and valued by researchers in the field of cold storage since the 1980s. In 1982, Tomlinson mentioned in his paper that an important way to improve the efficiency of the cold storage system is to improve the heat exchange efficiency between the refrigerant and the cold storage medium. In 1984, the direct contact heat transfer technology was applied to the ice storage system. The program was studied. This technique was later applied to gas hydrate systems. By comparing the technical and economic comparison of direct contact refrigeration systems with other cold storage systems, direct contact refrigeration systems have been found to have the highest efficiency. At the same time, it is pointed out that in order to make direct contact type cold storage into practical use, the following two problems must be solved. (1) Compressor: Commonly used engineering compressors are oil-lubricated compressors whose lubricating oil flows with the refrigerant throughout the refrigeration circuit. For the cold storage tank, the entry of the lubricating oil means that the cold storage tank is contaminated and affects the tank. Heat transfer, at the same time, it is impossible to return to the compressor after the lubricating oil enters the cold storage tank, which may cause the compressor to lose oil and be damaged. (2) Steam-water separation problem: The refrigerant vapor after heat exchange in the cold storage tank may bring a part of water vapor into the compressor and suck into the pipeline, which may damage the compressor. In 1984, Ternes described the first direct contact cold storage system established at the Oak Ridge National Laboratory in the United States. In view of the above two difficulties, the system uses an open cycle in its cold storage section, that is, instead of using a compressor in the system, the system is replaced by a hot water tank (before the condenser) and a recovery tank (after the cold storage tank). Successful operation was carried out using refrigerant R12, and gas hydrate formation was observed at the two phase boundaries (ie, the liquid-liquid boundary of the refrigerant and water, the vapor-liquid boundary). In addition, the paper also discusses the effectiveness of mechanical agitation and the reduction of the supercooling of the medium in the cold storage tank by the surface agent. In 1989, the research team led by Professor Mori of Keio University in Japan established a direct contact type cold storage test bench based on a large number of direct contact heat exchange research. The test bench has no cooling part, and the compressor adopts oil-free piston type. The compressor is equipped with a dryer after the cold storage tank. The research team conducted a systematic study on the refrigerants R12, R134a, R123, etc. After research, it was found that R12, R134a, etc. formed gas hydrates at the two interfaces, possibly Due to the large molecular diameter, R123 does not form a gas hydrate after heat exchange with water, but forms ice crystals. For the gas hydrate forming medium, the gas hydrate at the interface of the two phases is also different, the gas hydrate at the liquid D liquid interface is relatively tight, and the vapor-liquid interface generates loose porous gas hydration. Object. For additives or surface agents, they believe that as aluminum or zinc powder is added to the tank, the degree of subcooling required to form a gas hydrate will be reduced, while the addition of a nonionic surface agent such as Zony 1 fluorosurfactant, Unidyne The DS-401 requires less subcooling. However, an ice crystal nucleation bacterium DDDPseudomonas fluorescens has no effect on the reduction of subcooling. In the late 1980s, the University of Alabama in the United States established a direct contact type cold storage test rig that could provide both cooling and heating. The cooling test of the unit was to install an electric heater in the cold storage tank. The heating test is to install a coil in the cold storage tank. The heat supply is measured by the flow of cooling water in the coil. The compressor in the test bench is a conventional household compressor of 01186 kW. Two dry filters are applied before the compressor. In the device diagram, no oil separator was found, but in its conclusion, it is considered that a general oil-lubricated compressor can be used for a medium that does not chemically react with lubricating oil, which is beyond the meaning of general work. The quality is toxic or corrosive, oil-free compressors are still needed, and the accumulation of lubricating oil in the cold storage tank will also reduce the performance of the entire system. The working fluids tested on the test bench have R12 and R114. After the test, it is found that a control valve with better control should be installed in the system to prevent large fluctuations in flow.

Through the analysis of the above test phenomena, Mori and Isobe formed a gas hydrate for the evaporation of a single refrigerant droplet, and a physical model was proposed in which the vapor-liquid two-phase bubble covered by the hydrate film freely rises in the water. A mathematical model was established based on the heat balance between water and bubble heat transfer, heat of vaporization and hydrate formation heat. Gadalla et al. performed a numerical analysis of the temperature field in the cold storage tank.

The analysis results are consistent with the test.

2 Problems to be solved in the application research of direct contact type cold storage

Looking at the application research of direct contact type cold storage at home and abroad, although great progress has been made, we believe that the following problems still exist.

21 research objects

Almost all studies believe that gas hydrate systems are superior to ice storage systems because the phase transition temperature of gas hydrates is higher than freezing point and is compatible with air conditioning conditions. Previous studies on direct contact type cold storage were mainly for gases. In the study of hydrate systems, we believe that this view is one-sided. Although gas hydrates have the above advantages, they also have some fatal shortcomings that have not been solved so far, such as hydrate hydrate loosening in the vapor phase, on the one hand it will affect the amount of cold storage per unit tank volume, on the other hand, due to the vapor-liquid interface The accumulation of gas hydrates prevents the refrigerant gas from being discharged in time, which will prevent the cold storage from proceeding. In addition, if the cooling system is in direct contact with cooling, the gas hydrate will block the pipeline much more seriously than ice.

On the contrary, since most of the currently operating cold storage systems are ice storage systems, their technical conditions are much more mature than gas hydrate storage systems, namely direct contact ice storage systems and existing air conditioning units (especially cold storage air conditioning units). The adaptability is much stronger than that of gas hydrate systems. With the deepening of research on low temperature air supply technology, this advantage of direct contact ice storage system will gradually manifest itself in both technical and economic aspects. In addition, with the implementation of international CFC replacement, there are fewer and fewer types of refrigerants that can be used as cold storage media. When these refrigerants form ice crystals, they are either at a higher pressure or under negative pressure, and the pressure is too high. The cold storage tank is expensive to manufacture, and the negative pressure causes the outside air to leak into the system and the selection of the compressor is difficult. Therefore, the high pressure and low pressure refrigerant are adjusted to make the mixed working medium at normal pressure and normal temperature. An important research direction of cold storage technology.

22 test device aspects

As can be seen from the previous discussion, the main problems in the current test equipment are as follows: one is the compressor problem, and the price problem is solved. The best compressor used in the direct contact type cold storage system is undoubtedly an oil-free compressor. However, since the oil-free compressor is too expensive, the expensive price of the cold storage system developed on this basis will become the biggest obstacle in the future promotion and application. Therefore, oil-free compressors cannot be used in the experimental research. In the corresponding pipeline, the oil separator with high oil separation efficiency is used; the second is the steam-water separation device, because the water will react with the refrigerant and the lubricating oil, which will directly or indirectly affect the normal operation of the compressor, therefore, The system must be equipped with a steam-water separation device with high separation efficiency and simple operation; the third is a cooling system. In order to make the test device quick and practical, the cooling is the key link. Therefore, it is necessary to provide an air-conditioning device in the cooling system. Verify the practical effect of the cold storage system.

23 heat transfer aspects

The proposed system is based on direct contact heat exchange with high heat exchange efficiency, so it is necessary to pay attention to the study of heat transfer. Since the research on the direct contact type cold storage system has just started, the former continuation scholars pay more attention to the observation and study of the system phenomenon, and the heat transfer problem is less involved. Because of the cognitive reasons, the heat transfer problem of the direct contact ice storage cold The author has consulted the relevant literature and found no such literature.

24 theoretical aspects

In addition to Mori et al. established a theoretical model for gas hydrate formation, no other models have been found. For the direct contact ice storage problem, no theoretical research has been found. Therefore, in order to promote the smooth use of direct contact type cold storage, it is necessary to improve its theory and strengthen its research on heat transfer.

3 work we have done

The direct contact type cold storage research group of Shanghai University of Science and Technology analyzed the current status of foreign research on cold storage in this field. In response to the shortcomings in their research, a series of theoretical and experimental studies on direct contact ice storage were conducted, which is a direct contact cold storage technology. A gratifying step towards marketization. They mainly carried out the following work.

31 theoretical aspects

The direct contact ice storage system was analyzed. The results show that the direct contact type cold storage can save up to 30% compared with the traditional coil type cold storage system. According to the observation of this experiment and referring to the previous experimental results, the physical model of bubble generation is modified and the internal volume of the cold storage tank is established. The mathematical model of the heat transfer coefficient is the first to propose a physical model of mixed evaporative icing in a cold storage tank, and the volumetric heat transfer coefficient in the cold storage tank under icing conditions is obtained.

32 Established the first direct contact ice storage test system with both cold storage and cooling system at home and abroad. The system uses a general oil-lubricated fully enclosed compressor and uses molecular sieve adsorption and heating vacuum technology. The problem of the refrigerant carrying water in the cold storage system is solved. At the same time, certain oil separation technology is used to reduce the lubricating oil in the system to a small extent, and certain measures are taken to basically solve the ice blocking problem in the cooling system. The actual operation results show that the system is operating normally and meets the design requirements. This research fills the gap in China's field and makes the research of direct contact ice storage technology in China a leading position in the world, laying a solid foundation for China's international cooperation in this field. The system has the following functions: (1) Study on the law of ice crystal formation in the cold storage tank; (2) Study on the formation of refrigerant bubble in the cold storage tank; (3) Experimental study on heat transfer law in the cold storage tank; (4) Cool storage (5) Experimental study on the cooling law of the test bench; (6) Design and optimization study of the nozzle device; (7) Design and research of the refrigerant aqueous separation device; (8) Direct contact type cold storage medium Choose research.

33 The first heat transfer test of the cold storage tank in the direct contact type cold storage system

For the first time at home and abroad, the heat transfer test of the cold storage tank in the direct contact type cold storage system was carried out with the volumetric heat transfer coefficient as the research object. For the first time, a large number of experimental studies were carried out on the direct contact heat exchange between R123 and water forming ice crystals. In the tank, the water level in the tank, the temperature of the water at the beginning of the cold storage, the opening of the refrigeration throttle valve, the filling amount of the refrigerant in the refrigeration system, the type and arrangement of the nozzle, the type and quantity of the additive, etc. The influence of the thermal coefficient and the influence of the above factors on the temperature distribution in the cold storage tank were studied. Through experimental research, it is found that: (1) The internal heat transfer coefficient of the cold storage tank decreases with the decrease of the water temperature in the tank, and decreases with the increase of the water level in the tank. For the influence of the initial water temperature in the cold storage tank, the experimental study shows that the inner volume heat transfer coefficient of the tank decreases with the increase of the initial water temperature.

In addition, the larger the opening degree of the refrigeration throttle valve, the larger the heat transfer coefficient of the highest volume, and the larger the average heat transfer coefficient. This is mainly because the throttle valve can directly adjust the flow rate of the refrigeration system. The larger the opening degree, the larger the refrigerant flow rate and the greater the heat exchange amount. As can be seen from the test, the throttle opening degree is the same for water and The influence of the temperature difference of the refrigerant is relatively small, so the volumetric heat transfer coefficient is also larger. Studies have also shown that, just like in a typical refrigeration cycle, there is also a problem of optimal charge in the refrigerant in the cold storage cycle.

(2) For the nozzle, we found that the volumetric heat transfer coefficient in the cold storage tank increases with the increase of the number of nozzle holes, and decreases with the increase of the diameter of the nozzle hole. For the comparison of different nozzles under the total cross-sectional area of ​​the same nozzle hole, The volumetric heat transfer coefficient increases as the number of orifices increases and the diameter of the orifice decreases.

(3) The addition of different additives has different effects on the volumetric heat transfer coefficient in the cold storage tank. By comparison, the addition of the additive, iron powder, zinc powder and NaCl used in the test greatly increased the volumetric heat transfer coefficient in the cold storage tank, and the addition of butanol reduced the volumetric heat transfer coefficient in the cold storage tank.

34 proposed the concept of average exothermic efficiency

For the first time, the concept of average exothermic efficiency was proposed in the exothermic system. This concept reflects the state of the cooling process more realistically and applies the concept to study the influence of water flow. The results show that the average exothermic efficiency varies with time with water flow. For the same initial water temperature, the average exothermic efficiency will increase with the increase of water flow.

The derivation process of the volumetric heat transfer coefficient, the degree of influence of the volume heat transfer coefficient, and the mechanism and experimental operation results will be described in detail in the text.

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