Development of a microscale cryogenic device

In the World we live in today electronic devices like IR detectors, night vision equipment, thermal cameras, missile guiding system and superconductors operates better if not only in cryogenic temperatures. Developments in the electronics technology makes the devices become smaller. Therefore, cooling power that requires to keep them at cryogenic temperatures become also lower. Because of the lower cooling power requirements and reduced volume available, coolers are becoming more and more smaller. Therefore, microminiature Joule Thomson coolers are getting much more attention for as a cooling device because of their compact geometries, long life spans, not having moving parts and vibration free structure. In the thesis, a thermodynamic model is developed for cryocooler that has one and two cooling cycle. Using developed model, optimal geometry for a Joule-Thomson cryocooler is calculated by taking mass flow and dimensions as changing parameters. As a result, optimum geometries for two different types of cryocooler is found and changes in geometry and coefficient of performance of two different types are compared for same net cooling powers. Counter flow heat exchanger is an essential part for a Joule-Thomson cryocooler. As thermodynamics shows, the enthalpy of the working gas at the exit of the heat exchanger is required to be lower than the enthalpy at the compressor exit. If the enthalpy of the gas is higher, cooler loses ability the drawing heat from the environment. On the contrary, cooler starts to emit heat to it. The situation like this could cause catastrophic damage to the system. Because of this, ability of heat exchanger to heat up the gas exiting to the evaporator and cooling the gas that is going in is essential for a Joule-Thomson cryocooler. To be able to calculate cooling power and coefficient of performance (COP) of system, every component needs to be examined by using control volumes. Components that is examined in different control volumes are heat exchanger, restriction and evaporator. By applying first law of thermodynamics to these control volumes net cooling power and COP is founded. In the light of the energy equations of the system, it can be seen that COP value of the system can be increased either increasing the net cooling power or decreasing the required compressor power. All the irreversibilities in the cryocooler effects the cooling power and channel loss in the heat exchanger affects the required compressor power. To be able to comparing and summing all of the losses in the system at the same and find the optimized value for system geometry, entropy production of the system is required. Using the entropy production allows the consider parasitic heat losses and pressure drop in the same way while the losses has different kind of mechanism. Therefore, finding an optimized geometry for the desired cryocooler is possible through finding total entropy production. To be able to calculate the entropy production temperature profile of the heat exchanger is required. Thermodynamic cycle boundaries and material properties is need to be determined to be calculate a temperature profile After determining the general approach that is going to used, the systems working pressures are determined. Cooling temperature for the cooler is selected as 100 K and nitrogen is used as gas to be able to make the cooling in one stager Thus, the low-pressure value of the cycle is 6 bars. High-pressure value of the system is determined by comparing the finding COP of the system for cooler that has Carnot efficiency. 80 bars for system is taken as working pressure because COP is not increasing enough. There are two options while selecting the structure material of the heat exchanger. While a material that has high conductive heat transfer coefficient that maximizes the heat transfer rate of the heat exchanger, a material that has low conduction heat transfer coefficient decreases the conduction losses of the cryocooler. Working environment is a vacuum with has 0.005 Pa pressure and outer wall of the cryocooler is coated with a thin layer of gold to decrease the radiation heat transfer through the surfaces. Heat exchanger of cryocooler is divided to elements which have three sub elements named as high pressure, material and low pressure. A set of equations is created by energy balance of the three sub elements with each other and the environment. Using the equations models are developed for the two types of cryocooler. After inspecting the entropy production of the various geometry using developed thermodynamic model. It is seen that optimal geometries for cryocooler is different than expected. The reason of it is the not taking effect of the pressure drop in the calculation of the temperature profile of the heat exchanger because the friction loss in the channel decreasing the pressure and also increasing the temperature of the gas in reality. Therefore, some filter is required to include the effect of the pressure drop in the channel while inspecting the acquired data. The ratio of the power loss due to the friction in the channels to net cooling power is used to filter the calculated data. It is assumed that the calculated data is suitable to compare if the ratio is below or equal to 5%. Using this assumption correct values of optimized dimension for heat exchanger, restriction and evaporator is founded for two different types with various mass flow rate of nitrogen. Comparing the calculated data filtered by the assumption of channel loss divided by net cooling power needs to be under %5. Optimum dimensions are founded for a single cycle cryocooler for a cooling range from 13 mW to 400 mW. It can be see for cooling powers beyond the 40 mW channel's height and length is becoming fixed a value but channel width increases linearly. For lower cooling powers channel length is longer and channel height is higher because for lower cooling power the effect of the heat conduction on net cooling power is significantly higher so, to be able to decrease the conduction channel length needs to be longer. However, the longer the channel length the higher the losses in the channels. Thus, to be able to prevent the increase of loss height of the channel is needed to be higher. Using optimal geometries of the single cycle cryocooler as a starting point. The thermodynamic model developed for two cycle cryocooler which takes the effect of two cooling cycles one placed on top of the other with same amount of mass flow by using additional term for heat transfer through one cycle to another is used for determined the optimal geometry of the system for cooling powers between 80 mW to 750 mW. The total entropy production and COP values are found for various masses and geometries. Because of the heat transfer between two cycles losses in the channels are bigger than the single cycle, thus channel heights is bigger for two cycle cryocooler than single cycle one. Also, COP value of the cryocooler becomes slightly lower than single cycle for a same amount of mass flow going through in a cycle. However, the width of the channel for the two cycle cryocooler is significantly lower than to single cycle. In general, while approximately %4 decrease in COP is observed for two cycle. The width decreases up to %60 percent. In conclusion, it is clear that, with increasing cooling power width of the cryocooler is becoming higher due to its behavior to increase linearly with the net cooling power. Therefore, to be able to reduce the width and to get more compact design for cryocooler, two cycle cryocooler that has one cycle placed on another can be used. By sacrificing a little from the COP value, channel width of the system can be decreased dramatically. Which makes it proper for using it for higher cooling powers because of the same amount of cooling power with less cooling area which makes the cooling density higher in the cost of losing very small amount of COP by using the two cycle type cryocooler.