[Show abstract][Hide abstract] ABSTRACT: It is suggested that the housing of regenerators may have a significant impact when experimentally determining Nusselt numbers at low Reynolds and large Prandtl numbers. In this paper, a numerical model that takes the regenerator housing into account as a domain that is thermally coupled to the regenerator fluid is developed. The model is applied to a range of cases and it is shown that at low Reynolds numbers (well below 100) and at Prandtl numbers appropriate to liquids (7 for water) the regenerator housing may influence the experimental determination of Nusselt numbers significantly. The impact of the housing on the performance during cyclic steady-state regenerator operation is quantified by comparing the regenerator effectiveness for cases where the wall is ignored and with cases where it is included. It is shown that the effectiveness may be decreased by as much as 18% for the cases considered here. A reduced number of transfer units (NTU eff) is proposed based on the calculated regenerator effectiveness that accounts for the effect of the housing heat capacity.
International Journal of Heat and Mass Transfer 06/2013; 65:552-560. · 2.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cryosurgery is a medical technique that uses a freezing process to destroy undesirable tissues such as cancerous tumors. The handheld portion of the cryoprobe must be compact and powerful in order to serve as an effective surgical instrument; the next generation of cryoprobes utilizes precooled Mixed Gas Joule–Thomson (pMGJT) cycles to meet these design criteria. The increased refrigeration power available with this more complex cycle improves probe effectiveness by reducing the number of probes and the time required to treat large tissue masses. Selecting mixtures and precooling cycle parameters to meet a cryogenic cooling load in a size-limited application is a challenging design problem. Modeling the precooler and recuperator performance is critical for cycle design, yet existing techniques in the literature typically use highly idealized models of the heat exchangers that neglect pressure drop and assume infinite conductance. These assumptions are questionable for cycles that are required to use compact components. The focus of this research project is to understand how the cycle performance is impacted by transport processes in the heat exchangers and to integrate these findings into an empirically tuned model that can be used for mixture optimization. This effort is carried out through a series of modeling, experimental, and optimization studies. While these results have been applied to the design of a cryosurgical probe, they are also more generally useful in understanding the operation of other compact MGJT systems.A commercially available pMGJT cryoprobe system has been modified in order to integrate a suite of measurement instrumentation that can completely characterize the performance of the individual components as well as the overall system. Measurements include sufficient temperature and pressure sensors to resolve thermodynamic states, as well as flow meters in order to compute the heat and work transfer rates. Temperature sensors are also integrated within the recuperator in order to capture the spatially resolved heat transfer performance; these data are used to overcome the lack of correlations for heat transfer of the multi-phase mixture in the helically wound finned-tube heat exchanger. Test conditions were varied to achieve a range of temperatures, pressures, and thermodynamic qualities with mixtures of argon, R14 and R23. Recuperator and precooler conductance and pressure drop data for these test conditions are presented and fit to simple physics-based correlations; these correlations are integrated with an optimization model of the precooled MGJT cryoprobe that has been described in a previous paper. The predictive capabilities and optimal mixture selections of the model are compared with those of other models available in the literature, including the isothermal enthalpy difference model.
[Show abstract][Hide abstract] ABSTRACT: The “five-parameter model” is a performance model for photovoltaic solar cells that predicts the voltage and current output by representing the cells as an equivalent electrical circuit with radiation and temperature-dependent components. An important feature of the five-parameter model is that its parameters can be determined using data commonly provided by module manufacturers on their published datasheets. This paper documents the predictive capability of the five-parameter model and proposes modifications to improve its performance using approximately 30 days of field-measured meteorological and module data from a wide range of cell technologies, including monocrystalline, polycrystalline, amorphous silicon, and copper indium diselenide (CIS). The standard five-parameter model is capable of predicting the performance of monocrystalline and polycrystalline silicon modules within approximately 6% RMS but is slightly less accurate for a thin-film CIS and an amorphous silicon array. Errors for the amorphous technology are reduced to approximately 5% RMS by using input data obtained after the module underwent an initial degradation in output due to aging. The robustness and possible improvements to the five-parameter model were also evaluated. A sensitivity analysis of the five-parameter model shows that all model inputs that are difficult to determine and not provided by manufacturer datasheets such as the glazing material properties, the semiconductor band gap energy, and the ground reflectance may be represented by approximate values independent of the PV technology. Modifications to the five-parameter model tested during this research did not appreciably improve the overall model performance. Additional dependence introduced by a seven-parameter model had a less than 1% RMS effect on maximum power predictions for the amorphous technology and increased the modeling errors for this array 4% RMS at open-circuit conditions. Adding a current sink to the equivalent circuit to better model recombination currents had little effect on the model behavior.
Journal of Solar Energy Engineering 05/2011; 133. · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Computational fluid dynamic (CFD) analysis has been applied by various authors to study the processes occurring in the pulse tube cryocooler and carry out parametric design and optimization. However, a thorough and quantitative validation of the CFD model predications against experimental data has not been accomplished. This is in part due to the difficulty associated with measuring the specific quantities of interest (e.g., internal enthalpy flows and acoustic power) rather than generic system performance (e.g., cooling power). This paper presents the experimental validation of a previously published two‐dimensional, axisymmetric CFD model of the pulse tube and its associated flow transitions. The test facility designed for this purpose is unique in that it allows the precise measurement of the cold end acoustic power, regenerator loss, and cooling power. Therefore, it allows the separate and precise measurement of both the pulse tube loss and the regenerator loss. The experimental results are presented for various pulse tube and flow transition configurations operating at a cold end temperature of 80 K over a range of pressure ratios. The comparison of the model prediction to the experimental data is presented with discussion.
[Show abstract][Hide abstract] ABSTRACT: Regenerator models used by designers are macro-scale models that do not explicitly consider interactions between the fluid and the solid matrix. Rather, the heat transfer coefficient and pressure drop are calculated using correlations for Nusselt number and friction factor. These correlations are typically based on steady flow data. The error associated with using steady flow correlations to characterize the oscillatory flow that is actually present in the regenerator is not well understood. Oscillating flow correlations based on experimental data do exist in the literature; however, these results are often conflicting. This paper uses a micro-scale computational fluid dynamic (CFD) model of a unit-cell of a regenerator matrix to determine the conditions for which oscillating flow affects friction factor. These conditions are compared to those found in typical pulse tube regenerators to determine whether oscillatory flow is of practical importance. CFD results clearly show a transition Valensi number beyond which oscillating flow significantly increases the friction factor. This transition Valensi number increases with Reynolds number. Most practical pulse tube regenerators will operate below this Valensi transition number and therefore this study suggests that the effect of flow oscillation on pressure drop can be neglected in macro-scale regenerator models.
[Show abstract][Hide abstract] ABSTRACT: This paper reports on a micromachined Si/glass stack recuperative heat exchanger with in situ temperature sensors. Numerous high-conductivity silicon plates with integrated platinum resistance temperature detectors (Pt RTDs) are stacked, alternating with low-conductivity Pyrex spacers. The device has a 1 ?? 1-cm<sup>2</sup> footprint and a length of up to 3.5 cm. It is intended for use in Joule-Thomson (J-T) coolers and can sustain pressure exceeding 1 MPa. Tests at cold-end inlet temperatures of 237 K-252 K show that the heat exchanger effectiveness is 0.9 with 0.039-g/s helium mass flow rate. The integrated Pt RTDs present a linear response of 0.26%-0.30%/K over an operational range of 205 K-296 K but remain usable at lower temperatures. In self-cooling tests with ethane as the working fluid, a J-T system with the heat exchanger drops 76.1 K below the inlet temperature, achieving 218.7 K for a pressure of 835.8 kPa. The system reaches 200 K in transient state; further cooling is limited by impurities that freeze within the flow stream. In J-T self-cooling tests with an external heat load, the system reaches 239 K while providing 1 W of cooling. In all cases, there is an additional parasitic heat load estimated at 300-500 mW.
Journal of Microelectromechanical Systems 03/2010; · 2.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper reports on design, fabrication, and testing of a piezoelectrically actuated microvalve with integrated sensors for flow modulation at low temperatures. One envisioned application is to control the flow of a cryogen for distributed cooling with a high degree of temperature stability and a small thermal gradient. The valve consists of a micromachined die fabricated from a silicon-on-insulator wafer, a glass wafer, a commercially available piezoelectric stack actuator, and Macor ceramic encapsulation that has overall dimensions of 1.5 x 1.5 x 1.1 cm<sup>3</sup>. A piezoresistive pressure sensor and a thin-film Pt resistance temperature detector are integrated on the silicon die. The assembly process allows the implementation of normally open, partially open, and normally closed valves. At room temperature, gas flow modulation from 200 to 0 mL/min is achieved from 0- to 40-V actuation. Flow modulation at various temperatures from room temperature to 205 K is also reported. The pressure sensor has sensitivity of 356 ppm/kPa at room temperature, with temperature coefficient of sensitivity of -6507 ppm/K. The temperature sensor has sensitivity of 0.29 %/K. The valve and the sensors are tested across a wide range of temperatures, and the effect of temperature on performance is discussed.
Journal of Microelectromechanical Systems 09/2009; · 2.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper reports a Joule-Thomson cooling system that provides 0.1-1 W cooling power using a micromachined Si/glass perforated plate heat exchanger. The gas expansion is performed through a micromachined valve that is piezoelectrically actuated, or alternatively through a commercial jewel orifice. The modulated J-T system using the microvalve can achieve 254.5 K at a pressure difference of 430 kPa and 5-8 K temperature modulation at a given pressure. With a jewel orifice, the temperature at the expansion orifice drops 76.1 K from the inlet temperature for an inlet pressure of 1 MPa (145 psia) when the ethane mass flow rate is 0.269 g/s. The system can reach a lower temperature at 200.3 K in a transient state. The cooling power of the system is 200 mW at 228K and 1 W at 239 K, in addition to a parasitic heat load of 300-500 mW.
[Show abstract][Hide abstract] ABSTRACT: Over the past few decades, the pulse-tube cryocooler has advanced from a curiosity to one of the most attractive systems for providing reliable cryogenic cooling; it is now used in aerospace, medical and superconductor applications. This technology development has been enabled by the simulation tools that are available for regenerator, compressor, and inertance tube design. However, a dedicated design tool for the pulse-tube component in a pulse-tube cryocooler and the associated flow transitions between the pulse tube and the regenerator and the pulse tube and the inertance network is not currently available.
[Show abstract][Hide abstract] ABSTRACT: Distributed thermal loads are frequently encountered in large deployable structures used in space applications such as optical mirrors and focal plane electronics. An innovative mechanism for providing distributed cooling is an oscillatory pulse-tube cryocooler that is integrated with a fluid rectification system consisting of check-valves and buffer volumes in order to extract a small amount of continuous flow. This continuous flow allows relatively large loads to be accepted over a long distance. An additional advantage of the rectified system is the ability to provide rapid and precise temperature control via modulation of the flow rate in the flow loop. This paper presents an experimental demonstration of this temperature control method. A rectified interface is integrated with a thermal load and subjected to various thermal disturbances. Temperature regulation is enabled using temperature feedback control of a valve placed in the distribution loop. The control parameters are selected to meet temperature regulation specifications, including maximum temperature deviation and settling time in response to a step change in distributed load. The measured controlled transient behaviors to step and sinusoidal distributed load variations are presented. The predicted and measured behaviors are compared in order to validate the thermal model of the rectifying interface system.
[Show abstract][Hide abstract] ABSTRACT: This paper reports a micromachined recuperative heat exchanger integrated with in-situ temperature sensors. In this design, numerous high-conductivity silicon plates, integrated with platinum resistance temperature detectors (Pt RTDs) are fabricated and stacked alternating with low- conductivity Pyrex spacers. The device is intended for use with Joule-Thomson (J-T) coolers. The fabricated versions of the device have a footprint of 1 x 1 cm<sup>2</sup>, and lengths of up to 2 cm. Tests at cryogenic temperatures (120-210 K) show that the effectiveness of the heat exchanger is as high as 0.785. In a preliminary J-T self-cooling test, the temperature at the expansion orifice dropped 13 K from the inlet temperature for an inlet pressure of 527 kPa (76 psia). The integrated Pt RTDs presented a sensitivity of 0.26%/K in the linear operational range of 150-300 K, but remained usable at temperatures below that range. Details regarding evaluation apparatus are discussed.
[Show abstract][Hide abstract] ABSTRACT: Existing standards for testing the performance of flat-plate solar collectors are documented in ASHRAE 93 [ANSI/ASHRAE Standard 93-2003, 2003. Methods of Testing to Determine Thermal Performance of Solar Collectors, ISSN: 1041-2336, ASHRAE, Inc., 1791 Tullie Circle, Ne, Atlanta, GA30329], ISO 9806-1 [ISO Standard 9806-1:1994(E), 1994. Test Methods for Solar Collectors – Part 1: Thermal Performance of Glazed Liquid Heating Collectors Including Pressure Drop, ISO, Case Postale 56, CH-1211 Geneve 20, Switzerland] and EN12975-2 [European Standard EN12975-2:2001, 2001. Thermal Solar Systems and Components – Solar Collectors – Part 2: Test Methods, CEN, Rue de Stasart, 36, B-1050, Brussels]. The ASHRAE 93 standard requires an experimental determination of the steady-state collector efficiency under prescribed environmental conditions for a range of collector fluid temperatures. Each test requires a minimum of 20 min and 22 tests are required to fully characterize a collector’s thermal performance. The ASHRAE 93 testing procedure is further complicated by the fact that the prescribed weather conditions do not often occur in some locations, which prolongs the time required to conduct the performance tests for a given collector. The EN12975-2 collector test procedure provides an alternative transient test method that can be conducted over a larger range of environmental conditions. This paper compares the results obtained by applying the EN12975-2 standard with results obtained from the ASHRAE 93 steady-state tests for a well-designed single-glazed selective surface flat-plate collector. The collector thermal parameters, FR(τα)e and FRUL obtained by the two test methods show good agreement. The incident angle modifier coefficient determined in the ASHRAE method, which uses a separate test for this purpose, was found to be more accurate than that determined in the transient method according to the EN12975-2 standard, which obtains this value and all other collector parameters in the same step. This transient method, however, uses a refined collector model that includes specific terms for the wind speed dependence and the collector thermal capacitance, which are absent in the ASHRAE model. The long-term collector thermal performance as a part of a water heating system was simulated using the efficiency curves derived from each of the test methods. The solar fractions obtained by simulation are within 7% for both cases.
[Show abstract][Hide abstract] ABSTRACT: This paper describes a field experimental investigation of the effects of frost formation on the performance of a low-temperature large-scale evaporator coil used in industrial refrigeration systems. A series of experiments were conducted to determine the in situ coil cooling capacity of the evaporator over time as frost builds on its surfaces. Field-measured quantities include inlet and outlet air temperatures, inlet and outlet air relative humidity, and air volume flow rate. These measurements provide a baseline set of experimental data that can be used to validate numerical models of industrial evaporators operating under frosting conditions.
International Journal of Refrigeration-revue Internationale Du Froid - INT J REFRIG. 01/2008; 31(1):98-106.
[Show abstract][Hide abstract] ABSTRACT: This paper reports on a drug delivery system that provides modulated delivery of liquid-phase chemicals. The device uses silicon torsion springs on a 2times3 cm<sup>2</sup> chip to pressurize a soft polymeric reservoir and regulate flow with a piezoelectricaly actuated silicon microvalve that is 1.5times1.5times1 cm<sup>3</sup>. Using the finished device, regulated diffusion of a fluorescent dye into agar gel was demonstrated. Fluid flow out of the 500 muL reservoir could be regulated from 10-500 muL/min with up to 80 kPa of delivery pressure. Typical regulation consumes 0.136 muW of power. Analysis of the valve, reservoir springs, and a model based on pressure-enhanced diffusion are presented and are validated by experimental data.
[Show abstract][Hide abstract] ABSTRACT: This paper describes results from two types of micromachined recuperative heat exchangers intended for Joule-Thomson (J-T) cryosurgical probes, which require high stream-to-stream thermal conductance while restricting parasitic stream-wise (axial) conduction. In design A, rows of fins composed of high conductivity silicon are bonded onto a 100 mum thick base plate composed of low conductivity Pyrex glass. This planar device has a footprint of 6times1.5 cm<sup>2</sup> and 2.5 mm thickness, and is fabricated using a 5-mask process. In design B, numerous high-conductivity silicon plates alternating with low-conductivity Pyrex spacers are stacked together. This has a footprint of 1times1cm<sup>2</sup>, a length of 1.4 cm, and is fabricated using a 3-mask process. Preliminary experiments show that the primary performance constraint for design A is imposed by the compromise between mechanical robustness and transverse conductance of the thin glass base plate that separates the high pressure and low pressure streams. Design B enhances the robustness of the device and can sustain higher pressure.
[Show abstract][Hide abstract] ABSTRACT: This paper describes a normally open, self-encapsulated, gas valve that has embedded sensors for pressure and temperature monitoring. The valve has been validated at operating temperatures over 80-380 K, and at pressures up to 130 kPa. A perimeter augmentation scheme for the valve seat has been implemented to provide higher flow modulation. Two kinds of suspensions are described for the valve seat. In tests performed at room temperature, the flow was modulated from 980 mL/min. with the valve fully open (0 V), to 0 mL/min. with 60 V actuation, at an inlet pressure of 55 kPa. Cryogenic flow rate tests show similar modulation with flow from 166 mL/min. with the valve fully open, to 5.3 mL/min. with 120 V actuation voltage, at an inlet pressure of 70 kPa. The platinum RTD temperature sensor is independently tested from 40-450 K with sensitivity of 0.23 %/K in its operational range of 150- 450 K. The pressure sensor has sensitivity of 250 ppm/kPa at room temperature.
[Show abstract][Hide abstract] ABSTRACT: This paper describes a theoretical model of a large-scale, ammonia-fed evaporator coil used in an industrial refrigeration system and operating under low temperature air and refrigerant conditions that are typically encountered in refrigerated storage spaces. The model is used to simulate the performance of counter-flow and parallel-flow circuited evaporator coil designs under frosting conditions. The counter-flow frost model is validated using in situ data obtained from a field-installed evaporator coil. The performance of an evaporator in a parallel-flow circuit arrangement is simulated and compared to counter-flow circuiting. The effects of coil circuiting are evaluated in terms of the frost distribution across the evaporator coil and the associated reduction in cooling capacity during operation.
International Journal of Refrigeration-revue Internationale Du Froid - INT J REFRIG. 01/2007; 30(8):1347-1357.