Manfred Groll

Closed loop pulsating heat pipes: Part A: parametric experimental investigations

Abstract Closed loop pulsating heat pipes (CLPHPs) are complex heat transfer devices having a strong thermo-hydrodynamic coupling governing the thermal performance. In this paper, a wide range of pulsating heat pipes is experimentally studied thereby providing vital information on the parameter dependency of their thermal performance. The influence characterization has been done for the variation of internal diameter, number of turns, working fluid and inclination angle (from vertical bottom heat mode to horizontal orientation mode) of the device. CLPHPs are made of copper tubes of internal diameters 2.0 and 1.0 mm, heated by constant temperature water bath and cooled by constant temperature water–ethylene glycol mixture (50% each by volume). The number of turns in the evaporator is varied from 5 to 23. The working fluids employed are water, ethanol and R-123. The results indicate a strong influence of gravity and number of turns on the performance. The thermophysical properties of working fluids affect the performance which also strongly depends on the boundary conditions of PHP operation. Part B of this paper, which deals with development of semi-empirical correlations to fit the data reported here coupled with some critical visualization results, will appear separately.

Understanding operational regimes of closed loop pulsating heat pipes: an experimental study

Abstract Increasing performance of electronic components is resulting in higher heat flux dissipation. Two-phase passive devices are proven solutions for modern microelectronics thermal management. In this context, heat pipe research is being continuously pursued evolving newer solutions to suit present requirements. Pulsating heat pipes (PHPs), a relatively new and emerging technology is one such field of investigation. The operating mechanism of PHP is not well understood and the present state of the art cannot predict required design parameters for a given task. The aim of research work presented in this paper is to better understand these mechanisms through experimental investigations. Experiments were conducted on a PHP made of copper capillary tube of 2-mm inner diameter. Three different working fluids viz. water, ethanol and R-123 were employed. The PHP was tested in vertical (bottom heat mode) and horizontal orientation. The results strongly demonstrate the effect of input heat flux and volumetric filling ratio of the working fluid on the thermal performance of the device. Important insight into the operational regimes of the device has been gained.

Closed loop pulsating heat pipes Part B: visualization and semi-empirical modeling

Abstract Pulsating heat pipes have received growing attention from experimental and theoretical researchers in recent times. Behind its constructional simplicity lie the intriguingly complex thermo-hydrodynamic operational characteristics. Part A of this paper has presented the thermal performance results of a fairly large matrix of closed loop pulsating heat pipes. This paper, which is an extension of the previous work, first presents some more visualization results to highlight the complexities involved in mathematical formulation of the modeling problem. The phenomenological trends recorded in the visualization set-up are fully inline with the previous quantitative data of Part A. Critical review of the existing modeling approaches to CLPHPs is presented in the wake of these results. A detailed discussion follows on the important issues involved in the mathematical modeling of these devices. Then, semi-empirical correlations based on non-dimensional numbers of interest for predicting the thermal performance of CLPHPs are presented. Although there are limitations of the models presented herein, modeling by non-dimensional numbers seems to be most promising as compared to other existing techniques.

Heat and mass transfer in metal hydride reaction beds: Experimental and theoretical results ☆

Abstract A numerical model for transient heat and mass transfer within metal hydride reaction beds is presented. Theoretical predictions based on this model are compared with experimentally determined reaction rates and pressure-temperature distributions within the reaction beds. In addition, a mathematical treatment of the behaviour of coupled reaction beds is shown and as an example calculated hydrogen pressures are compared with experimental results.

An insight into thermo-hydrodynamic coupling in closed loop pulsating heat pipes

A Closed Loop Pulsating Heat Pipe (CLPHP) is a complex heat transfer device with a strong thermo-hydrodynamic coupling governing its thermal performance. To better understand its operational characteristics, a two-phase loop is constructed with a capillary tube (ID = 2.0 mm) having no internal wick structure. The loop is heated at one end and cooled at the other and partially made up of glass to assist visualization. The working fluid employed is ethanol. It is concluded from the study that a two-phase loop does represent the thermo-fluidic characteristics of a multi-turn CLPHP. Dynamic two-phase instabilities are present in a two-phase loop also; although the number of turns in a CLPHP increases the level of internal perturbations. The existence of an optimum number of turns for a given heat throughput requirement is explained. Also, it is shown that classical thermodynamics based on quasi-equilibrium theory seems not to be sufficient for complete system analysis. The performance (i.e., overall thermal resistance) is strongly dependent on the flow pattern existing inside the tubes. The role of gravity in the operation characteristics is clarified.

Performance comparison of vapour jet refrigeration system with environment friendly working fluids

A computer simulation of a vapour jet refrigeration system is carried out using a one-dimensional model based on mass, momentum and energy balances. The simulated performance of the system is in good agreement with the available experimental performance from the literature. A comparison of system performance is carried out for the same ejector geometry using the environmentally friendly working fluids R123, R134a, R152a and R717 (ammonia). The results suggest that, for different boiler temperatures, the entrainment ratio and the system efficiency (COP) depend mainly on the ejector geometry and the compression ratio.

Expanded graphite as heat transfer matrix in metal hydride beds

Abstract Metal hydrides can be used for a variety of applications. The thermal conductivity of metal hydride powders and the heat transfer coefficient between metal hydride powder beds and the walls of reaction beds are usually low. Heat transfer enhancement techniques have to be applied, when metal hydride powders are used. Here, expanded natural graphite/metal hydride compacts are described, possessing a high effective thermal conductivity (λ eff ≈19 W m −1 K −1 ) . Reaction kinetics of these compacts have been measured and compared to those of metal hydride beds employing Al-foams as heat transfer matrices. It was found that hydrogen absorption rates were only slightly decreased by the expanded natural graphite/metal hydride compacts, thus providing an efficient, cost-effective and simple solution.

Reaction beds for dry sorption machines

Abstract Periodically operating dry sorption machines utilize the reaction enthalpies of suitable reversible adsorption and absorption reactions for the generation of useful heat and cold. Typical working materials are ammoniated salts (chlorides in connection with ammonia), zeolites (with water), activated carbon (with methanol), metal hydrides (with hydrogen). The key elements of such machines are the reaction beds, typically cylindrical tubes, in which ad(ab)sorption and desorption takes place, or in which the ad(ab)sorbant is condensed and evaporated. The heat and mass transfer inside these reaction beds is decisive for the operational characteristics of respective machines. An overview is given of typical designs of the reaction beds. Special emphasis is given to heat transfer enhancement techiques inside the reaction beds. The external heat transfer from and to the reaction bed wall is also addressed.

Thermal control of electronic equipment by heat pipes

Abstract In the frame of the BRITE-EURAM european programme (KHIEPCOOL project), a literature survey on the main heat pipe and micro heat pipe technologies developed for thermal control of electronic equipment has been carried out. The conventional heat pipes are cylindrical, flat or bellow tubes, using wicks or axial grooves as capillary structures. In the field of micro heat pipes, three and four corner tubes have been developed. The pipes are mounted on single chips, in-line chip arrays or integrated into the component interconnection substrate. The best performances were achieved with Pleschs axially grooved flat miniature heat pipe [1], which is able to transfer a heat flux of about 60 W·cm −2 . Theoretical models have shown that the performance of micro heat pipe arrays increase with increasing tube diameter, decreasing tube length and increasing heat pipe density. The heat pipe technologies are classified and compared according to their geometry and location in the system. A list of about 150 references, classified according to their subjects, is presented.

Reaction kinetics in metal hydride reaction beds with improved heat and mass transfer

Abstract Periodically operating absorption engines, i.e. absorption heat pumps, heat transformers and refrigerators based on metal-hydrogen reactions, are becoming of great interest in technical applications. In order to keep the material inventory small and to design compact machines, short cycle times are necessary. In this paper, experimental investigations of the technical reaction kinetics of various AB5 alloys are presented and it is shown that it is equally important to improve both the hydrogen flow and the heat transfer in a reaction bed.