Valeri V. Vlassov

Experimental and theoretical investigation of a capillary pumped loop with a porous element in the condenser

Abstract A new CPL design is investigated experimentally and theoretically. In order to create a fixed physical interface between the liquid and the vapor phases inside the loop, the conventional tube condenser is replaced by a condenser containing a porous wick structure. The idea is to have a simple, light, and reliable system directed to applications where a high heat-transport capacity over long distances is needed, but a precise temperature control of the cold plate is not required. Experimental results, under different test conditions, are presented and illustrate the overall performance of the system. A CPL mathematical model, based on the nodal method, is described and validated experimentally.

Analysis of the operational characteristics and limits of a loop heat pipe with porous element in the condenser

This paper analyzes the operational characteristics and limits of a loop heat pipe (LHP) with flat porous elements in the condenser and evaporator. A mathematical model, based on the nodal method, was developed to simulate the thermal and hydrodynamic behavior of this specific type of LHP. Previously, the model was verified and validated by experiential data [I. Muraoka, F.M. Ramos, V. Vlassov, Int. Commun. Heat Mass Transfer 25 (1998) 1085]. The dry-out failure mechanisms of this device were investigated and its operational limits identified in terms of the heat load dissipated in the evaporator and the heat sink temperature at the condenser. Three distinct operational regimes were identified and characterized, taking into account the volume of vapor in the liquid core of the evaporator and the capillary pressure limit.

Heat Pipe Design Through Generalized Extremal Optimization

In this paper, an application of the Generalized Extremal Optimization (GEO) algorithm to the optimization of a heat pipe (HP) for a space application is presented. The GEO algorithm is a generalization of the Extremal Optimization (EO) algorithm, devised to be applied readily to a broad class of design optimization problems regardless of the design space complexity it would face. It is easy to implement, does not make use of derivatives, and can be applied to either unconstrained or constrained problems with continuous, discrete, or integer variables. The GEO algorithm has been tested in a series of test functions and shows to be competitive to other stochastic algorithms, such as the Genetic Algorithm. In this work, it is applied to the problem of minimizing the mass of an HP as a function of a desirable heat transport capability and a given temperature on the condenser. The optimal solutions were obtained for different heat loads, heat sink temperatures, and three working fluids: ammonia, methanol, and ethanol. The present design application highlights the GEO features of being easily implemented and efficient on tackling optimization problems when the objective function presents design variables with strong nonlinear interactions and is subject to multiple constraints.

Modeling of a Loop Heat Pipe for Ground and Space Conditions

This paper presents a mathematical model of a LHP, where its results were compared with experimental test results and the LHP behavior was predicted for Space conditions. In the LHP condenser, the condensate film thickness in the tube is determined by the solution of the conjugate equations of energy, momentum and mass balance in the control volume. Evaporator and compensation chamber are both described by a few transient nodes with generalized key parameters, whose values were adjusted by test results. Experimental LHP consists of cylindrical stainless steel evaporator with an integral compensation chamber, where the primary wick is of micropore UHMW polyethylene and the condenser is an aluminum plate, with acetone as the working fluid. The tests conditions have been reproduced in the mathematical model and its parameters were adjusted using the measured temperatures. The model was applied to predict the LHP behavior for Space conditions, corresponding to the circular low Earth equatorial orbit.

Generalized extremal optimization for solving complex optimal design problems

Recently, Boettcher and Percus [1] proposed a new optimization method, called Extremal Optimization (EO), inspired by a simplified model of natural selection developed to show the emergence of Self-Organized Criticality (SOC) in ecosystems [2]. Although having been successfully applied to hard problems in combinatorial optimization, a drawback of the EO is that for each new optimization problem assessed, a new way to define the fitness of the design variables has to be created [2]. Moreover, to our knowledge it has been applied so far to combinatorial problems with no implementation to continuous functions. In order to make the EO easily applicable to a broad class of design optimization problems, Sousa and Ramos [3,4] have proposed a generalization of the EO that was named the Generalized Extremal Optimization (GEO) method. It is of easy implementation, does not make use of derivatives and can be applied to unconstrained or constrained problems, non-convex or disjoint design spaces, with any combination of continuous, discrete or integer variables. It is a global search meta-heuristic, as the Genetic Algorithm (GA) and the Simulated Annealing (SA), but with the a priori advantage of having only one free parameter to adjust. Having been already tested on a set of test functions, commonly used to assess the performance of stochastic algorithms, the GEO proved to be competitive to the GA and the SA, or variations of these algorithms [3,4]. The GEO method was devised to be applied to complex optimization problems, such as the optimal design of a heat pipe (HP). This problem has difficulties such as an objective function that presents design variables with strong non-linear interactions, subject to multiple constraints, being considered unsuitable to be solved by traditional gradient based optimization methods [5]. To illustrate the efficacy of the GEO on dealing with such kind of problems, we used it to optimize a HP for a space application with the goal of minimizing the HPs total mass, given a desirable heat transfer rate and boundary conditions on the condenser. The HP uses a mesh type wick and is made of Stainless Steel. A total of 18 constraints were taken into account, which included operational, dimensional and structural ones. Temperature dependent fluid properties were considered and the calculations were done for steady state conditions, with three fluids being considered as working fluids: ethanol, methanol and ammonia. Several runs were performed under different values of heat transfer

Modeling of a Loop Heat Pipe with Evaporator of Circumferential Vapor Grooves in Primary Wick

A new configuration of internal geometry of LHP evaporator is investigated. Cylindrical evaporator has circumferential vapor escape channels instead of axial channels, commonly used in evaporators; only one axial groove is presented to provide vapor collection and direction to the vapor line. These circumferential grooves are combined with circumferential micro-threads on evaporator case internal surface. Mathematical model of LHP has been developed with emphasis on simulation of heat-mass transfer in evaporator. The model was verified by the test results obtained for the LHP with axial-groove wick evaporator. The model was then modified and applied to the new evaporator wick geometry. Comparative studies reveal the main reason for considerable improvement of the performance for new configuration: significant reduction of the two-phase layer between wall and wick. Two mechanisms are found to be able to contribute in such thickness reduction. There is effect of liquid trapping in the microthreads, and like-heat-pipe effect along circumferential grooves.

New concept of space radiator with variable emittance

A new concept of space radiator of variable emittance for satellite thermal control is presented. The radiator is composed of two stages which exchange heat through radiation between finned surfaces covered with variable emittance coatings, whose emissivity is increased with temperature. Under cold conditions the radiative heat coupling between the stages is minimal, preventing the equipment subcooling, while in hot conditions the heat exchange is increased. A steady-state mathematical model was developed and numerically coupled to an optimization algorithm, and a design optimization procedure was performed. Two optimization criteria were employed: minimization of the radiator mass and the power consumption of heater under cold conditions. The Generalized Extremal Optimization algorithm was used as the optimization tool. The design was then modeled in detail using the SINDA/Fluint package considering orbital conditions of the EQUARS satellite. The performance of the radiator concept proposed here was also compared to a conventional design, for the same operational conditions. It is envisioned that the utilization of such radiators in micro-satellites will lead to considerable electric power savings for safe heaters and may contribute to a longer satellite life. In the design trade-off, the cost for this saving is additional radiator mass and volume.

Design Optimization of Loop Heat Pipes with Cylindrical Evaporator and Integral Reservoir for Space Application

Design optimization of a LHP system for a space application is considered. The system is composed of the LHP itself, an interface with the heat source (saddle) and a radiator. The criterion is minimal system mass while meeting the operational requirements. The optimization is performed with simultaneous consideration of hot and cold conditions with respect to imposed heat loads to the evaporator and external heat fluxes over the radiator panel. The design parameters of the system optimized are the active length of the evaporator, internal and external diameters of the primary wick, volume and size of the reservoir, thickness and width of the saddle, diameters and tube thickness of the transport lines and condenser, length of the condenser, dimensions of the radiator panel and the amount of the LHP working fluid charged. The LHP mass and optimal design parameters are obtained for three working fluids: ammonia, propylene and acetone; a comparative study of the optimal mass characteristics is performed. Fixe...

Experimental investigation of natural convective heat transfer from a round plate of complex configuration

Abstract Experimental results of free convective heat transfer on a round plate with additional elements on the surface at ambient air temperatures are presented. The tests were performed at horizontal and vertical positions of the plate. The influence of an annular non-heated barrier and partial upper insulation was studied. Experimental data were obtained at steady-state and transient modes over the Rayleigh number range from 1×107 to 2×108. Validity of the quasi-steady-state postulation for transient heat transfer was evaluated. Results were reduced to power-type relations Nu=f(Ra) and compared with the known steady-state data for free convection over smooth surfaces.

Analysis of Concept Feasibility and Results of Numerical Simulation of a Two-Stage Space Radiator With Variable Emissivity Coating

ABSTRACT We present analytical models and a detailed numerical model of a two-stage space radiator with variable emissivity coating on its internal surfaces and we correlate them with experimental data to validate the numerical model. The feasibility of the radiator concept called VESPAR (Variable Emittance SPAce Radiator) is proven using an analytical model and confirmed by a numerical model. This numerical model is then used to compare the thermal performance of VESPAR and a conventional flat radiator. The comparison confirmed that the VESPAR radiator for satellite thermal control could lead to power savings for survival electric heaters or even its elimination.