Experiment

Experiment file

TPX.html

Experiment

In-orbit demonstration of two-phase heat transport technology

Primary investigator

A.A.M. Delil

Contact point

National Aerospace Laboratory NLR, The Netherlands

Category

Fluid Flow and Heat Transfer Science

Main research area

Two-phase heat transport

Abstract

Two-phase thermal management systems utilise the latent heat of vapourisation of the working fluid, and therefore require smaller components and less power than conventional single-phase systems do. Two-phase heat transport is a promising technology for efficient thermal management of future complex spacecraft.

In the STS-60 flight, February 1994, NASA's Space Shuttle Discovery carried TPX, the Two-Phase flow eXperiment, conducted for the European Space Agency (ESA) by a Dutch-Belgian consortium led by NLR. The purpose of the TPX experiment was to demonstrate heat transport by means of two-phase loops under space conditions and to compare in-flight data with terrestrial test data and thermal modelling predictions.

TPX Two-Phase eXperiment hardware

The experiment was carried out in a so-called Get Away Special Container, a standardised, sealed cylindrical container capable of accommodating payloads up to 90 kg, 500 mm diameter and 700 mm length. These containers are mounted to the Shuttle cargo bay.

TPX integrated in GAS container in space shuttle cargobay

After a start command given by the Shuttle crew, the experiment has run autonomously for 43 hours.

Experiment schematic

Heat, supplied to two parallel capillary pumped evaporators (a flat one and a cylindrical one), causes evaporation of the working fluid, sets the mass flow rate and generates the pumping pressure to maintain the working fluid circulation in the system. The heat, extracted from the fluid in the condenser sections and the subcooler, is radiated to space via the GAS canister lid. The control of the temperature set-point of the loop is done by a Peltier element controlling a thermal accumulator (reservoir), which contains liquid and vapour in equilibrium. In addition, the loop contains two vapour quality sensors, a controllable three-way valve with a vapour bypass line, and depriming heaters for the two evaporators. The complete experiment had to fulfil the GAS canister requirements and restrictions, such as allowable volume, mass (maximum 90 kg total), no power and data communication connections to STS (hence limited battery energy and internal data storage), and limited crew action (only on/off commands). Moreover, shuttle dependent GAS thermal sink conditions needed to be taken into account.

GAS thermal sink conditions

The TPX baseline had to meet the many objectives for the different experiment constituents: capillary pumped loop (CPL), vapour quality sensors (VQS) and multichannel condensers, each of them being a scaled-down version of the concept originally developed for power systems up to 10kW. The downscaling should not affect the objectives of the in-orbit demonstration of these concepts.

The present experiment used capillary instead of mechanical pumping to drive the flow of the liquid from condenser back to evaporator. Capillary pumping avoids vibrations and is therefore capable of meeting stringent requirements to microgravity disturbance level, temperature stability and controllability.

Equipment

The fluid circuit included capillary evaporators and an accumulator developed by SABCA, vapour quality sensors developed by NLR and BE, condensers developed by FSS and a three-way valve developed by BE. The two-phase loop was equipped with various sensors and actuators for the thermal control and the acquisition of test data. A small computer for control and data acquisition was part of the flight hardware.

TPX and Electronic Ground Support Equipment

Objectives

An experiment objective was to assess and compare the (limits of the) CPL heat transport capability under low gravity and terrestrial conditions and to compare these with predictions resulting from thermal modelling using the ESA developed general thermal analyzer program ESATAN. An additional objective was the use of the low-g and terrestrial TPX test data and the outcomes of testing in the NLR two-phase test loops, in order to verify the NLR approach for the thermal- gravitational scaling of two-phase flow and heat transfer.

The Capillary Pumped Loop related objectives were to demonstrate in a low-gravity environment:

• The capability of a CPL to transport heat in a smooth and continuous way.
• Proper CPL operation for different heat loads applied to two evaporators in parallel.
• The capability of a CPL to share heat load.
• The capability to prime evaporators by controlled management of the reservoir fluid content.
• The capability to start-up from low temperatures.
• The capability of a CPL to adjust and maintain a set-point temperature.

A cylindrical and a flat evaporator were present to determine in low-g:

• Transport capabilities and maximum pumping pressures.
• Depriming behaviour and repriming capabilities, by controlled accumumlator actions.
• Evaporator heat transfer coefficients.
• Interaction between parallel evaporators, including heat load sharing.

Incorporation of two Vapour Quality Sensors (VQS) had several objectives:

• To prove the feasibility of the concept in space.
• To demonstrate the proper performance of the sensors accommodating ammonia, the most promising working fluid for space-oriented two-phase systems.
• To compare the performance of the two sensors in order to assess the influence of the location within the loop and of small construction differences.
• To perform calibrations and to assess differences in low-g and terrestrial sensor performances (very important for the design of future space-oriented quality sensors).
• To carry out quality control exercises to prove the usefulness of quality sensors for system control and to demonstrate proper performance of the Swalve (controllable 3-way valve).

The multichannel condenser design objectives can be summarised by demonstration and verification of the working principle and determination of the performance limits in a low-gravity environment.

Results

TPX has successfully flown; many objectives are met. The TPX design has proven to be a good balance between the consequences of the very limited amount of flight opportunities and conflicting requirements. Theoretical models for two-phase flow & heat transfer have been partly confirmed, others are to be revised and low gravity flow pattern maps are to be created (based on additional flight data). The viability of two-phase heat transfer technology has been proven under 0-g, however more in-orbit investigations are to be carried out.

A re-flight of TPX is foreseen to obtain more flight experience needed in satellite programmes, especially for Earth observation (ENVISAT-2, EOS).

Literature reference

A.A.M.Delil et al., TPX for in-orbit demonstration of two-phase heat transport technology: evaluation of flight & post-flight experiment results, NLR TP 95191 U and SAE 951510.

Platform

Space Shuttle Discovery STS-60

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