Mode Transition Analyses of the Attached Pressurized Module Cabin Air Loop with EcosimPro

The change in mode status of the Attached Pressurized Module (APM), termed a mode transition, is due to the need of changing the APM configuration triggered by nominal or contingency events, i.e: initial system activation and further de/reactivation, payload activation, crew, ground or station initiated mode changes, etc. Past simulations of the APM Cabin Air Loop, for individual operational modes, have been performed by Dornier. This paper presents the results of the hydraulic and thermal analyses of the APM Cabin Air Loop for mode transition with the new version of the European Space Agency (ESA) supported software EcosimPro. The range of analysis has now been extended to long duration simulation of transitions between modes, which was impractical in the past. The transitions cover both operational and non-operational modes, with rapid changes in boundary conditions, as heat loads, air and coolant mass flows, crew metabolic heat, International Space Station (ISS) supplied air interface temperature and relative humidity, etc. INTRODUCTION The APM is one of the major elements of the Columbus Orbital Facility (COF) programme, which represents the uropean contribution to the ISS (Fig.1). The APM is designed for multi-disciplinary Payload missions as micro-gravity, life science, earth observation, space science and technology disciplines. The APM Environment Control and Life Support Subsystem (ECLSS) provides a pressurized environment during module transportation to orbit and for on-orbit operation when docked to the Space Station with related crew operations. Figure 1: APM attached to the International Space Station (ESARAD model made by J.A. Romera) The ECLSS provides the following functions: • Atmosphere pressure monitoring and control (cabin depressurization, positive/negative pressure relief, total pressure monitoring, and O2 and CO2 partial pressure monitoring) • Cabin Air Temperature and Humidity control (cabin ventilation, temperature and air humidity monitoring, air temperature control, air dehumidification and Inter Module Ventilation (IMV)) • Air revitalization (air sampling and air filtering) APM • Fire detection and suppression • Vacuum and venting • N2 supply • Control of PAM shell and ECLSS heaters CABIN LOOP FUNCTIONAL DESCRIPTION The main objectives of the Cabin loop are: a) collection of heat loads from crew and radiative and convective heat loads from surrounding surfaces (racks, end cones, standoffs) and transfer to the Thermal Control Subsystem (TCS) via the Condensing Heat Exchanger (CHEX). b) collection of humidity produced by the crew and potential water leakages in the Cabin by the CHEX, and the transfer of the condensed water to the ISS for further re-use. c) Revitalization of the Cabin air by returning the Cabin air to the ISS via the IMV, for CO2 removal and O2 partial pressure control. The functional schematic of the Cabin Loop is shown in Fig. 2. Figure 2: Cabin Loop Functional Schematic The APM air is sucked from the deck-floor and partly given via IMV to ISS for revitalisation and partly pumped by the Cabin fan to the Cabin loop supply duct where it is mixed with the IMV supply air. Downstream the mixture of the IMV supply air with the air coming from the cabin fan is passed to the CHEX. After passing the filter the air flow is splitted and flows through the two parallel cores of the heat exchanger, from which one is cooled by the TCS water loop and the other is not cooled and serves as a bypass. The mass flow ratio of the bypass and main-stream is controlled by the Temperature Control Valve (TCV). The TCV adjusts the air outlet temperature by mixing the cold main air stream with the warm bypass air. The valve is externally controlled by the Cabin Temperature Control Unit (CTCU), according to the selected Cabin air temperature set point (between 18 and 27 °C). The air flowing out of the CHEX is distributed throughout the Cabin via the two branches of the Cabin loop supply ducting and the Cabin diffusers located in the upper cabin corners (four on each side alternating with lights). The minimum air flow rate is 51 m/hr per diffuser. The humidity produced by the crew is condensed in the active core of the CHEX. The condensate is collected in the slurper header, which interfaces to the slurper line. The water separator sucks an air-condensate mixture out of the CHEX through the slurper line. This mixture is separated into pure water and air. The water is driven through the condensate line to the ISS, and the air is returned to the loop at the outlet of the CHEX. DEFINITION OF TRANSITION MODES The APM Activation and Operations Modes Analysis (Ref.1), describes the APM mission in terms of phases and modes, and the initial activation of the APM. The mission phases describe the entire lifetime of the APM from Ground Processing, Launch and Ascent, Initialization, Routine Operations up to the APM Safe Disposal. Within the overall mission scenario and its associated phases the purpose of having APM modes is to provide a standard method for describing complex flight configurations and provide the ability to reliably operate the APM in those configurations. The main Operational APM modes are: • Passive Mode • Unberthed Survival Mode • Berthed Survival Mode • Stand-by Mode • Housekeeping • Support Mode Unmanned/Manned • Nominal Mode Unmanned/Manned The change in mode status is termed a mode transition and is in response to a need to change the APM configuration status reflecting either a nominal or contingency event (see sketch below). Nominal mode transitions will be triggered by different events, e.g.: initial system activation, system de/reactivation, payload activation or crew, ground or station initiated mode changes SELECTION of TRANSITION MODES From all possible permutations of transition modes as described in Ref.1 (see Fig. 3), four sequences of transitions have been retained for further analyses with EcosimPro: Figure 3: Modes Transition Case 1: Nominal Manned (3 crew) -> Support Unmanned (No crew) -> Housekeeping (2 crew) -> Support Manned (3 crew) -> Nominal Manned (3 crew) Case 2: Nominal Manned (3 crew) -> Standby (No crew) -> Support Manned (3 crew) -> Nominal Manned (3 crew) Case 3: Nominal Manned (3 crew) -> Berthed Survival (No crew) -> Support Manned (3 crew) -> Nominal Manned (3 crew) Case 4: Nominal Manned (3 crew) -> Nominal Unmanned (No crew) -> Nominal Manned (3 crew) As no timing has been specified to each mode, we have assumed durations ranging from 4 hours (Support Unmanned and Manned and Housekeeping), to 8 hours (Berthed) and to 16 hours (Nominal Unmanned). In order to stress the system, crew metabolic activity is always set to maximum (corresponding to heavy work figures of Ref. 2, with the exception of housekeeping mode where they correspond to the little work figures). Because the way the crew component is treated in EcosimPro, the total metabolic heat generated (sensible plus latent heat dissipation) is given as input. EcosimPro formulation of the crew component computes the sensible and latent heat produced, based on the environmental variables of the cabin. EcosimPro APM Cabin Air Loop Model EcosimPro is a completely new version of an original software tool developed within the framework of a contract with ESA for simulation of environmental control and life support systems on manned spacecrafts. The first version ran under Unix system and was specially created with a dedicated and comprehensive library of components, which included pipelines, valves, fans, heat exchangers, PID controllers, crew, etc. The Cabin Air Loop model of the APM was originally created by Dornier (Ref. 3) as part of the design activities of the APM ECLS Subsystem. Instabilities and convergence problems associated to simulations with rapid changes in boundary conditions (mass flow, metabolic heat, crew number, etc.) were found during first trials on Mode transitions. Some of those problems were connected to mathematical accuracy of the software solver, which induced very long computing time (in the order of four hours of CPU per each hour of simulation time). Others, as for example those dealing with the stop of fans, were due to inherent model configuration fault. At that time the software developer Empresarios Agrupados International (EAI), was working with the beta version of the new EcosimPro for PC, which offered a much faster and efficient solver (up to 10 times improvement as compared to the Unix computation speed). Before the new version could be used for the APM Cain Loop Air model, some tasks already foreseen under ESTEC Contract 11044/94 (Adaptation of current ECLS components to the new ECOSIM 3.0), had to be performed which requested the technical support of EAI: • Conversion, update and validation of the ECLSS Library • Conversion of DORNIER’s model, as there is no model commonality from old to new EcosimPro versions • Validation of the APM converted model with respect to previous Unix model • Upgrade of the APM model to improve its mathematical stability and accuracy in fast transient cases, and its adaptation to special mode The sketch of the modified APM model using SmartSketch (flow-sheet editor of EcosimPro) is shown below: Figure 4: APM Cabin Air Loop model The upgraded APM model has more than 1300 variables and more than 500 equations.