Thermal Storage for High-Power Small Satellites

As the power levels and sizes of small satellites grow, new capabilities become possible along with new challenges for thermal control. Greater amounts of heat must be transported across longer distances, making it more difficult to control component temperatures using simple, passive systems. This paper describes the performance of an innovative thermal storage technology for small satellite thermal control systems. The thermal storage unit helps maintain temperature stability by efficiently incorporating a solid/liquid phase-change material (PCM). This paper describes the results of an analysis and testing program that proved the feasibility of the PCM thermal storage concept. We formulated a simple model for a high-power small satellite in an orbital thermal environment. We found that proper selection of the PCM depends on the thermal environment, thermal control system characteristics, and characteristics of the thermal load. The model shows that a properly designed thermal storage system can dramatically reduce temperature variation. We designed and built a sub-scale PCM thermal storage unit and measured its performance with a heat pipe under conditions that simulate operation in a small satellite thermal control system. Results of these tests demonstrate the capability of the thermal control system to reduce temperature variation during transient operation

Rac Activation Induces NADPH Oxidase Activity in Transgenic COSphox Cells and Level of Superoxide Production is Exchange Factor-Dependent

Transient expression of constitutively active Rac1 derivatives, (G12V) or (Q61L), was sufficient to induce phagocyte NADPH oxidase activity in a COS-7 cell model in which human cDNAs for essential oxidase components, gp91phox, p22phox, p47phox, and p67phox, were expressed as stable transgenes. Expression of constitutively active Rac1 in “COSphox” cells induced translocation of p47phox and p67phox to the membrane. Furthermore, translocation of p47phox was induced in the absence of p67phox expression, even though Rac does not directly bind p47phox. Rac effector domain point substitutions (A27K, G30S, D38A, Y40C), which can selectively eliminate interaction with different effector proteins, impaired Rac1V12-induced superoxide production. Activation of endogenous Rac1 by expression of constitutively active Rac-guanine nucleotide exchange factor (GEF) derivatives was sufficient to induce high level NADPH oxidase activity in COSphox cells. The constitutively active form of the hematopoietic-specific GEF, Vav1, was the most effective at activating superoxide production, despite detection of higher levels of Rac1-GTP upon expression of constitutively active Vav2 or Tiam1 derivatives. These data suggest that Rac can play a dual role in NADPH oxidase activation, both by directly participating in the oxidase complex and by activating signaling events leading to oxidase assembly, and that Vav1 may be the physiologically relevant GEF responsible for activating this Rac-regulated complex

Surface characterization of a real-world cylinder liner subject to deposition from combustion

This paper investigates the effects of combustion product deposition using a cylinder liner taken from a C-segment passenger vehicle run for 105,000 miles. Using a novel methodology of Atomic Force Microscopy and X-ray Photoelectron Spectroscopy the pressure coefficient of boundary shear strength of asperities and the nature of the depositions along the liner is considered to predict the boundary friction of a piston ring pack. Results show that the combustion depositions create localized values of the pressure coefficient of boundary shear strength of asperities at top dead centre, mid-stroke and bottom dead centre, increasing ring pack friction by 50 N in the combustion stroke per engine cycle

An investigation into the oil transport and starvation of piston-ring pack

In order to accurately predict the lubricant film thickness and generated friction in any tribological contact, it is important to determine appropriate boundary conditions, taking into account the oil availability and extent of starvation. This paper presents a two-dimensional hydrodynamic model of a piston ring pack for prediction of lubricant film thickness, friction and total power loss. The model takes into account starvation caused by reverse flow at the conjunctional inlet wedge, and applied to a ring-pack, comprising a compression and scraper ring. Inlet boundaries are calculated for an engine cycle of a 4-cylinder, 4-stroke gasoline engine operating at 1500rpm with conditions pertaining to the New European Drive Cycle (NEDC). The analysis shows that the two main sources of starvation; firstly due to a physical lack of inlet meniscus and secondly due to reverse flow at the inlet wedge, significantly affect the prevailing conditions from the generally assumed idealised boundary conditions. Such an approach has not hitherto been reported in literature

An Investigation into the Oil Transport and Starvation of Piston-Ring Pack

In order to accurately predict the lubricant film thickness and generated friction in any tribological contact, it is important to determine appropriate boundary conditions, taking into account the oil availability and extent of starvation. This paper presents a two-dimensional hydrodynamic model of a piston ring pack for prediction of lubricant film thickness, friction and total power loss. The model takes into account starvation caused by reverse flow at the conjunctional inlet wedge, and applied to a ring pack, comprising a compression and scraper ring. Inlet boundaries are calculated for an engine cycle of a four-cylinder, four-stroke gasoline engine operating at 1500 r/min with conditions pertaining to the New European Drive Cycle. The analysis shows the two main sources of starvation: first, due to a physical lack of inlet meniscus and second, due to reverse flow at the inlet wedge significantly affecting the prevailing conditions from the generally assumed idealised boundary conditions. Such an approach has not hitherto been reported in literature

Atomic force microscopic measurement of a used cylinder liner for prediction of boundary friction

Accurate simulation performs a crucial role in the design and development of new modern internal combustion engines. In the case of piston rings, simulations are used to effectively predict generated friction and power loss of proposed designs. These are consequences of viscous shear of a thin lubricant film, likewise boundary friction caused by direct interaction of piston rings with the cylinder liner/bore surface. The most commonly used model for determining boundary friction is that of Greenwood and Tripp. The model requires the pressure coefficient of boundary shear strength of asperities from the softer of the contacting surfaces as an input. This parameter needs to be measured. The paper describes the process of measurement using an Atomic Force Microscope (AFM), both for a dry surface and that wetted by the presence of a lubricant layer. For realistic results, the investigated specimen is a used, tested engine cylinder liner where boundary active lubricant additives are bonded to its surface as well as combustion products. This approach is as opposed to the previously reported works using new flat surfaces with base oil or partially formulated lubricants, and has not previously been reported in literature. The results show that for used cylinder liners, the measured boundary shear strength of asperities varies according to location along the stroke. Results are reported for the Top Dead Centre, Mid-stroke and Bottom Dead Centre locations. The measurements are subsequently used with 2D Reynolds Solution for a top compression ring-liner contact, where it is found that accurate localised predictions of generated friction and power loss can be made instead of the usual average value approach reported in literature

Boundary friction characterisation of a used cylinder liner subject to fired engine conditions and surface deposition

In cylinder friction contributes as a primary source of parasitic dissipations in IC engines. For future engines to become more efficient, with enhanced fuel economy and increased power output, accurate prediction of new designs is required over the full lifetime of an engine. The work carried out presents use of a local pressure coefficient of boundary shear strength of asperities value, taking into account the localised effects of surface texture, coating and surface deposition. XPS spectra analysis was also carried out to identify the surface depositions as a result of combustion, not previously taken into account during piston ring pack simulation. Friction was shown by simulation to drop by up to 30% between the compression and combustion stroke as a result of using a carriable coefficient of boundary shear strength of asperities. It was found that piston varnish on the liner corresponded to higher values of the pressure coefficient of boundary shear strength of asperities, therefore showing the importance of using real system components run under representative operating conditions or numerical analyses

Results of measured data from atomic force microscope on ring pack performance

Frictional losses of an IC engine include 40-50% contribution due to piston assembly-liner conjunction. Reduction of friction would improve fuel efficiency and decrease harmful emissions. Therefore, it is important to accurately predict the frictional losses due to viscous shear of a thin lubricant film as well as boundary friction, generated by the direct contact of real rough contiguous surfaces. Greenwood and Tripp model is used to evaluate the contribution due to boundary friction. The model requires the determination of pressure coefficient of boundary shear strength of asperities, ς, which is analogous to the asperity coefficient of friction. This should be determined through measurement, using Atomic Force Microscopy (AFM) in Lateral Force Mode (LFM). The value of ς is dependent on the combination of surface and lubricant as a system. Boundary active lubricant additives adsorb or bond to the surface asperities and affect the value of ς. The value of this coefficient also alters with the evolution of interacting surfaces through the process of wear as well as any degradation of the lubricant. The approach can be used to create a database of such values for different lubricant-surface systems, in particular for piston-liner interactions

Effect of cylinder de-activation on the tribological performance of compression ring conjunction

The paper presents transient thermal-mixed-hydrodynamics of piston compression ring-cylinder liner conjunction for a 4-cylinder 4-stroke gasoline engine during a part of the New European Drive Cycle (NEDC). Analyses are carried out with and without cylinder de-activation (CDA) technology in order to investigate its effect upon the generated tribological conditions. In particular, the effect of CDA upon frictional power loss is studied. The predictions show that overall power losses in the piston-ring cylinder system worsen by as much as 10% because of the increased combustion pressures and liner temperatures in the active cylinders of an engine operating under CDA. This finding shows the down-side of this progressively employed technology, which otherwise is effective in terms of combustion efficiency with additional benefits for operation of catalytic converters. The expounded approach has not hitherto been reported in literature

Disease severity adversely affects delivery of dialysis in acute renal failure

Background/Aims: Methods of intermittent hemodialysis (IHD) dose quantification in acute renal failure (ARF) are not well defined. This observational study was designed to evaluate the impact of disease activity on delivered single pool Kt/V-urea in ARF patients. Methods: 100 patients with severe ARF (acute intrinsic renal disease in 18 patients, nephrotoxic acute tubular necrosis in 38 patients, and septic ARF in 44 patients) were analyzed during four consecutive sessions of IHD, performed for 3.5-5 h every other day or daily. Target IHD dose was a single pool Kt/V-urea of 1.2 or more per dialysis session for all patients. Prescribed Kt/V-urea was calculated from desired dialyzer clearance (K), desired treatment time (t) and anthropometric estimates for urea distribution volume (V). The desired clearance (K) was estimated from prescribed blood flow rate and manufacturer's charts of in vivo data obtained in maintenance dialysis patients. Delivered single pool Kt/V-urea was calculated using the Daugirdas equation. Results: None of the patients had prescription failure of the target dose. The delivered IHD doses were substantially lower than the prescribed Kt/V values, particularly in ARF patients with sepsis/septic shock. Stratification according to disease severity revealed that all patients with isolated ARF, but none with 3 or more organ failures and none who needed vasopressive support received the target dose. Conclusion: Prescription of target IHD dose by single pool Kt/V-urea resulted in suboptimal dialysis dose delivery in critically ill patients. Numerous patient-related and treatment-immanent factors acting in concert reduced the delivered dose. Copyright (C) 2007 S. Karger AG, Basel