1、 Harrison Skye is a PhD graduate student in the Department of Mechanical Engineering, University of Wisconsin - Madison, Madison, WI. Sanford Klein and Gregory Nellis are professors in the Department of Mechanical Engineering at the University of Wisconsin Madison, Madison, WI Experimental Apparatus
2、 for Measuring the Performance of a Precooled Mixed Gas Joule Thomson Cryoprobe Harrison M. Skye Sanford A. Klein, PhD Gregory F. Nellis, PhD Student Member ASHRAE Fellow ASHRAE Member ASHRAE ABSTRACT Cryosurgery is a technique for destroying undesirable tissue such as cancers using a freezing proce
3、ss. A previous ASHRAE paper describes the development of a thermodynamic modeling tool for a precooled Mixed Gas Joule-Thomson (MGJT) cryoprobe used for cryosurgery. An experimental test facility has been constructed to measure the performance of a precooled MGJT cryoprobe; the experimental data wil
4、l be used to tune and verify the model, and to demonstrate additional cooling capacity available with the optimal mixture compositions and operating parameters selected by the model. A commercially available cryoprobe system has been modified to integrate measurement instrumentation that is sufficie
5、nt to characterize the performance of the individual components as well as the overall system. Measurements include temperature and pressure sensors to resolve thermodynamic states, and flow meters to calculate heat and work transfer rates. A thermal load is applied using an electric heater to chara
6、cterize the refrigeration performance. Temperature measurements located inside of the recuperator are used to capture the heat transfer performance of the two-phase, multi-component mixture. An uncertainty analysis for the experiment is presented which shows that the performance targets can be compu
7、ted from the measurements with an uncertainty of less than 10% under nominal operating conditions using both a synthetic refrigerant and hydrocarbon based gas mixture. Preliminary data for a mixture of R23, R14 and argon are reduced and presented in order to demonstrate the computation of various pe
8、rformance metrics. INTRODUCTION Cryosurgery is a technique for destroying undesirable tissue using a freezing process. The procedure can used to ablate prostate and liver cancer tumors and it is also used in a variety of procedures in dermatology, gynecology, and cardiology Rubinsky 2000. Modern cry
9、oprobes are typically energized by Joule-Thomson (JT) cycles; the use of a gas mixture working fluid, rather than a single component such as nitrogen, greatly increases the refrigeration capacity of the JT cycle Brodyansky et al., 1971 and thus the size of the cryolesion produced by the cryoprobe. M
10、ixture optimization techniques for JT cycles in general have been described by Gong et al. 2000, Alexeev et. al 1997, and (Keppler 2004) have been developed specifically for JT cryoprobes Maytal et al. 2006, Fredrickson et al. 2006, and Gong et al. 2000. Mixed Gas Joule Thomson (MGJT) cycles with pr
11、ecooling (i.e. a system where the high pressure gas mixture is cooled prior to entering the recuperator by a second refrigeration system) provide additional cooling compared to the single stage MGJT cycle Alexeev et al. 1999. A pre-cooled MGJT cryoprobe is the focus of this paper. The cryoprobe feat
12、ures a Giaque-Hampson LV-11-C043356 ASHRAE Transactions2011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either
13、 print or digital form is not permitted without ASHRAES prior written permission.style recuperator with approximately 50” length of 0.05” ID finned stainless steel tubing helically wound about a mandrel. A previous ASHRAE transactions paper Skye et al. 2008, RP 1472 describes a model of the precoole
14、d MGJT system and captures the fundamental thermodynamic and heat transfer processes that govern system performance. This model can be used to select optimal mixture compositions and other operating parameters that maximize cooling power given hardware size limitations. An experimental apparatus des
15、cribed here captures the performance of a precooled MGJT cycle. One purpose of the experiment is to collect data that will be used to tune and verify the model. Another objective is to demonstrate the additional cooling that can be attained using new mixtures selected by the optimization model. A co
16、mmercially available cryosurgical probe system shown in Figure 1 was disassembled in order to integrate measurement instrumentation sufficient to determine the performance of individual components as well as the overall system. The modifications enable more detailed measurements than the gross perfo
17、rmance measurements (i.e., tip temperature and load) that are otherwise available with the cryoprobe. For example, Figure 1 highlights the tip of the cryoprobe, which encloses the expansion valve and is the active section of the probe (i.e., the cold section that provides the refrigeration used to f
18、orm the cryolesion). The thermodynamic states at locations before and after the expansion valve, as well as at a location downstream of the refrigeration load are critical to the system performance. However, the unmodified probe configuration does not allow for measurement of the temperature and pre
19、ssure at these states. The modifications enable direct measurements at these locations and therefore these and other thermodynamic states can be resolved. Temperature and pressure measurements are used to measure the thermodynamic performance of each of the components in the cryoprobe system. It is
20、important to empirically characterize all of the components in order to allow comparison with the detailed model, as accurately predicting the performance of a gas mixture in any of the components of the refrigeration cycle is particularly difficult. The accuracy of the correlations used to predict
21、the thermodynamic properties of mixtures at Additional PRTs are located within the low pressure side of the recuperator and are labeled “PRTi#” in Figure 2(a). The mass flow rate is measured in both cycles using calorimetric flow meters; these measurements are labeled 1stm and 2ndm for the precoolin
22、g and MGJT cycles, respectively, and are used to quantify heat and work transfer rates. An interchangeable orifice (a precise jewel orifice from Bird Precision that is 0.01 inch thick and has an opening diameter that varies depending on the test but nominally ranges between 0.01 inch and 0.02 inch)
23、and a bypass valve are used to independently regulate the pressure ratio and mass flow applied to the MGJT cycle. Heat is applied to simulate a biological thermal load using a Nichrome wire heater, which is labeled according to the associated voltage and current measurements, Vloadand Iload. Finally
24、, the cold components in the cycle are covered in MultiLayer radiation Insulation (MLI) and enclosed within a vacuum facility to minimize the parasitic heat leak. Figure 2(b) shows thermodynamic state points of the MGJT cycle for a nominal operating condition using a mixture with a molar composition
25、 of 9.1% Argon, 40.9% R14, and 50.0% R23 (Mix 001) in the JT cycle, overlaid on a pressure-enthalpy (P-h) diagram for the mixture computed using the NIST4 or SUPERTRAPP Ely 1992 mixture database. recuperator and precooler enclosureactive portion of cryoprobe tipfluid lines to compressorshandleFigure
26、 1 Picture of handheld cryoprobe showing the locations of the active (cold) portion of the tip as well as the recuperative and precooling heat exchangers. 2011 ASHRAE 357Figure 3 shows the instrumentation for the temperature measurements located within the low pressure side of the recuperator (label
27、ed “PRTi” in Figure 2); two small PRTs are embedded at diametrically opposed positions at each of four axial locations in a cryogenic grade G-10 (fiberglass-resin composite) sheath that slides over the helically wound finned-tube recuperator. The measurements provide the flow stream temperature dist
28、ribution as the PRTs are in direct contact with the fluid and the G-10 sheath has a very low conductivity (and therefore limits axial conduction). A ninth PRT shown in the far left of Figure 3 labeled “free PRT” measures the low pressure stream temperature before the finned section of the recuperato
29、r. The temperature measurements within the recuperator are used to quantify the heat transfer performance including the pinch point temperature (i.e., the minimum temperature difference between the hot and cold streams) and the spatially resolved conductance. The temperature profile is useful in ide
30、ntifying physical explanations for poor performance that may include: (1) temperature profiles that do not enable efficient or compact heat exchange, (2) early dry-out (i.e. two-phase flow becomes entirely vapor) causing low heat transfer coefficients, or (3) excessive pressure drop in the low press
31、ure stream yielding higher low pressure stream temperatures and subsequently less heat transfer in the recuperator. Thermodynamic states for the data shown in Figure 2(b) are computed using three different methods (A, B, C) depending on the measurements that are available at the particular location.
32、 Method A is used to determine the thermodynamic states at locations where direct temperature and pressure measurements are both available. Measurements computed using Method A (1,3,5-7) are shown as circles and indexed with boxed numbers. Method B is used to determine the thermodynamic state at loc
33、ations where direct temperature measurements are available but pressure measurements are not. Measurements computed using Method B (B1-B5) occur within the low pressure side of the recuperator and are indicated with triangles. The pressure at these states is computed by applying a linear pressure dr
34、op (i.e. the pressure drop is assumed to be equal between each of the temperature sensor locations within the recuperator) between states 7 and 1 (where the pressure is measured). Method C is used to determine the thermodynamic state at locations where neither direct temperature nor pressure locatio
35、ns are available. Measurements computed using Method C (C1-C5) occur within the high pressure side of the recuperator and are indicated by squares. The hot stream enthalpy values are computed at these states using an energy balance for the sections of the recuperator between the temperature sensors
36、in the low pressure stream and the pressures are computed by applying a linear pressure drop between states 4 and 5. Note that the recuperator high pressure inlet state represented by the boxed “4” in Figure 2(b) (not indexed in Figure 2(a) is not measured, but rather is computed using the recuperat
37、or section energy balances, and the pressure is assumed to be halfway between states 3 and 5 (where pressure is measured). UNCERTAINTY ANALYSIS An uncertainty analysis was performed to ensure that the resolution of the instrumentation installed in the test facility could capture the performance of t
38、he overall system with adequate fidelity. The model described in Skye et al. 2008 was used to carry out the uncertainty analysis using nominal operating parameters as well as accuracies of readily available sensors and gas mixtures. Cryoprobe refrigeration (loadQ), precooling, recuperative, and tota
39、l heat exchanger conductances ( pcUA,recUA , and total pc recUA UA UA=+), and the cryoprobe compactness target (load totalQUA, which is the figure of merit that, to first order, determines the cryoprobe size Skye et al. 2008) are used as metrics to examine the test facility instrument accuracy. The
40、analysis was performed for both hydrocarbon (HC) and synthetic refrigerant (SR) based gas mixtures, which nominally represent those that will be tested in the MGJT cycle; the SR and HC mixtures used for this analysis are shown in Table 1. The precooling VC cycle working fluid is assumed to be R22. N
41、ominal operating parameters and sensor accuracies are shown in Table 2 where the measurements correspond to state points in Figure 2; note that some of the measurements in Figure 2(a) are not required to fully constrain the overall system model from Skye et al. 2008 and therefore are not included in
42、 Table 2. The measurements not included in the uncertainty analysis will be used for quantifying individual component performance rather than overall system performance targets. The detailed calculation of the uncertainty is not presented here due to space limitations, but will be available in the f
43、inal report prepared for this project 358 ASHRAE Transactions(RP-1472). The results of the uncertainty analysis are presented in Table 3; both the heat exchanger conductances (UA) and the cryoprobe compactness target (load totalQUA) can be measured with 10% or less experimental uncertainty. 1stmPRT
44、9P9TC 8PRT 7PRT 1TC 3P3P8P7Vload , Iload+-vacuum jarbypass valvebypass valveP1PRTijewel orifice 2ndmPRT 5P5PRT 6MGJTVCrecuperatorprecooler(a) (b) Figure 2 (a) Schematic of experimental test facility including measurement instrumentation integrated with the MGJT cryoprobe system. (b) Nominal JT cycle
45、 state points overlaid on a P-h diagram for the gas mixture where the #s correspond to the thermodynamic states indicated in (a). G10 fiberglass sheath PRTs embedded in G10recuperatorPRT wires“free”PRTG10 sheath covers recuperatorrecuperator high pressure exit, to PRT 5precoolerrecuperator high pres
46、sure inlet, state 4Figure 3: Picture of the G-10 sheath and embedded PRTs that measure the temperature profile in the low pressure stream of the recuperator. 2011 ASHRAE 359EXPERIMENTAL RESULTS Preliminary data have been collected using this test facility for two different mixture compositions inclu
47、ding the manufacturers original proprietary mixture and Mix 001. The data are recorded when the system achieves steady state, an operating condition established by minimal fluctuations of the computed recuperator conductance; the recuperator is thermally massive compared to the other components and
48、therefore is the final component to achieve steady operation. These data are useful for demonstrating the thermodynamic and heat transfer performance metrics that can be reduced from the data and used to tune the MGJT cryoprobe optimization model Skye et al. 2008. A key component of the optimization
49、 model is the computation of thermodynamic property data for the multi-component mixtures at cryogenic temperatures. The precise measurements presented here are ideal for evaluating the capability of the computed property data to predict thermodynamic phenomena occurring the cycle. Specifically, the isenthalpic expansion process across the orifice in the MGJT cycle can be characterized by the Joule-Thomson effect temperature change (TJT). A comparison of the predicted and measured TJTis presented in Figure 4, where the predicted values are co
