Boil-off Calorimetry Research Articles

EXPERIMENTAL STUDY ON THE FACTORS AFFECTING THE INSULATION PERFORMANCE OF FLAME-RETARDANT MULTILAYER INSULATION MATERIALS

Flame-retardant multilayer insulation materials act as effective thermal insulation blankets of cryogenic containers that store flammable and explosive cryogenic liquids. This study used standard static liquid nitrogen boil-off calorimetry to test the insulation performance of eight groups of flame-retardant multilayer insulation materials with different wrapping parameters. The effects of four factors, namely the layer density, seaming process, number of reflector layer, and variable-density multilayer insulation arrangement, on the insulation performance were analysed. Three layer densities were considered: 4.47, 3.08, and 2.50 layers/mm. Two types of seaming processes were discussed: the overlapped and fold-over seaming processes. Three numbers of reflector layers were considered: 60, 70, and 80. Two variable-density multilayer insulation arrangements with similar thicknesses were discussed: 10-10-40 and 20-20-20 layers of reflectors allocated for low-, medium- and high-density segments. The conclusions are as follows: Decreasing the layer density enhances the performance of multilayer insulation; Using the fold-over seaming process results in less heat flux and lower apparent thermal conductivity; An increase in the number of reflector layers weakens radiative heat transfer, resulting in better thermal insulation; Furthermore, for a given wrapping thickness, reducing the number of reflectors appropriately in low- and medium-density segments improves the insulation performance; Optimizing and controlling the layer density of each density segment are also essential for variable-density multilayer insulation effects. This study provides supporting theories and reference data for practical engineering applications.

Boil-off calorimetry enthalpy measurements and equation of state of an aqueous pyridine azeotrope

We present measurement and modeling of the thermo-physical properties of an aqueous pyridine azeotrope for evaluation as a working fluid for power and thermal management applications. The enthalpy values of the Pyridine/water azeotrope were measured using an electrical matching boil-off flow calorimeter in the range of P = 0.101 to 11 MPa and T = 373 to 575 K. A Peng-Robinson equation of state model for mixtures was fitted to the experimental data using a temperature-dependent binary interaction parameter. The results were used to develop a pressure-enthalpy diagram with isotherms in both the liquid and vapor states.

Macroflash boiloff calorimetry instrument for the measurement of heat transmission through materials

The Macroflash is a flat plate boiloff calorimeter that provides effective thermal conductivity (ke) data for a wide range of materials from thermal insulation to structural composites to ceramics. The apparatus and method provide a practical, standardized way to measure heat transmission through materials under steady-state conditions at below-ambient temperatures and under different compressive loads. Another unique feature of this device is that it can provide test data at both large and small temperature differences. Using liquid nitrogen as a method to directly measure the heat flow rate, the device is applicable to testing under an ambient pressure environment at a wide range of temperatures, from 77 K to 403 K. Test specimens may be isotropic or non-isotropic; homogeneous or non-homogeneous. The Macroflash is currently calibrated in the range from approximately 10 mW/m-K to 1,000 mW/m-K using well-characterized materials. Reference data for hundreds of test specimens including foams, powders, aerogels, plastics, composites, carbon composites, wood, glass, ceramic, metal, and multi-layered composites have been compiled from Macroflash testing. The Macroflash apparatus is described and its operation, instrumentation, and control system discussed. The calibration approach is detailed as well as analysis of key data sets of standard materials.

Investigations of Heat Transfer in Vacuum between Room Temperature and 80 K

The heat transfer between room temperature and 80 K is controlled using various insulating material combinations. The modes of heat transfer are well established to be conduction and thermal radiation when in a vacuum. Multi-Layer Insulation (MLI) in a vacuum has long been the best approach. Typically this layered system is applied to the cold surface. This paper investigates the application of MLI to both the cold and warm surface to see whether there is a significant difference. In addition if MLI is on the warm surface, the cold side of the MLI may be below the critical temperature of some high temperature superconducting (HTS) materials. It has been proposed that HTS materials can serve to block thermal radiation. An experiment is conducted to measure this effect. Boil-off calorimetry is the method of measuring the heat transfer.

Cylindrical boiloff calorimeters for testing of thermal insulation systems

Cryostats have been developed and standardized for laboratory testing of thermal insulation systems in a cylindrical configuration. Boiloff calorimetry is the measurement principle for determining the effective thermal conductivity (ke) and heat flux (q) of a test specimen at a fixed environmental condition (boundary temperatures, cold vacuum pressure, and residual gas composition). Through its heat of vaporization, liquid nitrogen serves as the energy meter, but the design is adaptable for various cryogens. The main instrument, Cryostat-100, is thermally guarded and directly measures absolute thermal performance. A cold mass assembly and all fluid and instrumentation feedthroughs are suspended from a lid of the vacuum canister; and a custom lifting mechanism allows the assembly and specimen to be manipulated easily. Each of three chambers is filled and vented through a single feedthrough for minimum overall heat leakage. The cold mass design precludes direct, solid-conduction heat transfer (other than through the vessel's outer wall itself) from one liquid volume to another, which is critical for achieving very low heat measurements. The cryostat system design details and test methods are discussed, as well as results for select thermal insulation materials. Additional cylindrical boiloff calorimeters and progress toward a liquid hydrogen apparatus are also discussed.

Flat-plate boiloff calorimeters for testing of thermal insulation systems

Cryostats have been developed and standardized for laboratory testing of thermal insulation systems in a flat-plate configuration. Boiloff calorimetry is the measurement principle for determining the effective thermal conductivity (ke) and heat flux (q) of test specimens under a wide range of actual conditions. Cryostat-500 is thermally guarded to measure absolute thermal performance when calibrated with a known reference via an adjustable-edge guard ring. With liquid nitrogen as the energy meter, the cold boundary temperature can be adjusted to any temperature between 77 K and approximately 300 K by the interposition of a thermal resistance layer between the cold mass and the specimen. A low thermal conductivity suspension system has compliance rods that adjust for specimen thickness and compression force. Material type, thickness, density, flatness, compliance, outgassing, and temperature sensor placement are important test considerations, and edge effects and calibration techniques for the apparatus are crucial. Over the full vacuum pressure range, the thermal performance capability is nearly four orders of magnitude. The horizontal configuration provides key advantages over the vertical cylindrical cryostats for testing at ambient pressure conditions. Cryostat-500’s design and test methods, other flat-plate boiloff calorimeters, and results for select thermal insulation materials (composites, foams, aerogels) are discussed.

Warm-up calorimetry of Dewar-Detector Assemblies

Boil-off isothermal calorimetry of Dewar-Detector Assemblies (DDA) is a routine part of their Acceptance Testing Procedure. In this approach, the cryogenic liquid coolant (typically LN2) is allowed to naturally boil-off from the Dewar well to the atmosphere through a mass flow meter; the parasitic heat load is then evaluated as the product of the latent heat of vaporization and the "last drop" boil-off rate.An inherent major limitation of this technique is that it may be performed only at the fixed boiling temperature of the chosen liquid coolant. A further drawback is related to the explosive nature of "last drop" boiling, manifesting itself as an uneven flow rate. This especially holds true for advanced High Operational Temperature Dewar-Detector Assemblies, typically featuring short cold fingers and working at 150 K and above.In this work, we adapt the well-known technique of dual-slope calorimetry and show how accurate heat load evaluation may be performed by comparing the slopes of the warm-up thermal transients under different trial added heat loads. Because of the simplicity, accuracy and ability to perform calorimetry literally at any temperature of interest, this technique shows good potential for replacing traditional boil-off calorimetry.

Coupling currents losses bench mark test of ITER subsize conductor

To solve the discrepancies frequently observed between coupling currents losses results measured on similar samples at different labs, a bench merit test action has been carried out among five laboratories of the four partners of the ITER project. The specimen for the bench mark action is a short sections of Nb/sub 3/Sn cable-in-conduit conductor with the same layout as the last but one stage of the ITER conductor. The five labs measure the AC losses by boil-off calorimetry. The test programme, selected according to the capabilities of the test equipment, allows a direct comparison of the test results over a broad range of field pulses.

ボイル・オフ・カロリメトリー法による熱量測定

Very low boil-off of cryogens is required for cryostats containing superconducting magnets used in various fields. It is important that the thermal insulating efficiency is improved. In order to accurately design the thermal insulating efficiency of components for cryostats, it is necessary to measure the emissivity of the thermal shielding plates and the average thermal conductivity of the multilayer insulation and supports. The boil-off calorimetry method has often been used for measurements of these low temperature heat transfer characteristics. In this review, applications of the boil-off calorimetry method and details of measurement are summarized.

Theory of Boil-Off Calorimetry

The usual formulation used to compute heat flow in boil-off calorimetry ignores the sensible heating of the vapor in the calorimeter. A formulation which includes this heating is derived; a single temperature measurement (of the vapor leaving the calorimeter) permits this effect to be computed. As the required temperature measurement has not been given for reported experiments, the vapor heating effect cannot be computed directly for those cases. Therefore, an expression for the computation of this temperature is derived theoretically and applied to reported experiments. It is shown that, even in optimally designed calorimeters, the sensible heating of the vapor can be appreciable, and that the use of the usual formulation can introduce significant error in the computed heat flow. Finally, it is indicated how this error can explain, at least partially, the Heat Paradox which has been observed in insulation and Dewar evaluation tests.