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Radiant floor heating systems have become a reference technology, but their use for cooling purposes has proven inconvenient in many applications due to their reduced cooling capacity and condensation issues. Nonetheless, potentialities and drawbacks of radiant floor cooling systems have been frequently addressed and simulated, given the large potential advantages of employing a single emissive system for all seasons. This paper aims to provide a comprehensive review of the modeling methods for radiant floor cooling systems proposed in scientific papers and also used in simulation software and technical standards. Models are classified according to their characterizing features, and the distinctive contributions of each method are discussed. Additionally, the modeling of the most relevant phenomena affecting floor performance is further discussed. The review revealed the presence of two main modeling classes, one only focusing on the floor's conductive heat transfer and the other integrating active floor analysis within the building's energy model of the thermal zone. Despite the presence of many modeling methods that are able to consider the most important effects in the radiant cooling system operation, not all the phenomena present in a practical application are fully described. Therefore, there is an ongoing need for more comprehensive, possibly easily characterizable, modeling approaches. [ABSTRACT FROM AUTHOR]

This paper deals with the problem of an infinite cylindrical heat source embedded into a saturated porous medium and subject to a cross-axial Darcian flow. Only forced convection is considered. We derived the transient dimensionless solution through a combined analytical – numerical method consisting of four steps: (a) a preliminary dimensional analysis of the constitutive equations of the problem in order to find the dimensionless groups governing the solution; (b) the identification of the validity range of the model as a function of the just-mentioned dimensionless groups; (c) the numerical resolution of the problem; (d) the synthesis of the numerical results in a general dimensionless form. Specifically, we provide several dimensionless maps of the 2D thermal field evolution for six different orders of magnitude of the Péclet number ( 10 - 3 – 10 2 ) . The evolution of the temperature of the heat source is fully illustrated and discussed through plain dimensionless criteria. Then, we discuss the time, space and fluid velocity scales in which the solution is practically equivalent to the ones given by a linear heat source and a purely conductive model. We conclude that the present model has to be employed to evaluate the temperature in proximity of the heat source when the reference Péclet number is greater than 0.5. On the contrary, the linear model can be successfully used for radial distances 5–10 times greater that the heat source radius, depending on the reference Péclet number. [ABSTRACT FROM AUTHOR]

The heat transfer performance of an ionic jet impinging on the upper surface of a heated plate is investigated. Ions are injected by a point, set at high voltage. The working fluid is the viscous dielectric ester MIDEL 7131, a refrigerant for power transformers. Different tests are performed on the working fluid, varying the composition, the shape and the polarity of the point, the applied voltage, the point-to-plane distance, and the imposed heat flux. Heat transfer coefficients are augmented up to 310% with respect to natural convection, a higher enhancement than the one obtained with traditional transformer oil. The electrohydrodynamic flow can be generated without significant power input (<0.01 W in these experiments). Cooling of power transformers can be greatly enhanced by this technique. [ABSTRACT FROM AUTHOR]

In the present work, we investigate the effect of the electrical and gravitational force fields on weakly forced convection (Reynolds numbers range from 500 to 5600) in a square duct heated from the bottom side. On the top side, we placed an array of sharp point electrodes. Microgravity conditions were obtained during a parabolic flight campaign. At the application of a sufficiently-high DC electric field, a plume-like motion is induced in the fluid by the mechanism of ion injection and heat transfer is dramatically augmented. The working fluid is the dielectric liquid HFE-7100. Local temperatures on the heated wall were measured by liquid crystal thermography and by electrical resistance thermometers. By means of the temperature field, we were able to characterize the behaviour of the ionic jets and their interaction with the crossflow. We also investigated the effects of the Reynolds number, the gravity level, and the major electrical parameters on the average heat transfer coefficients. When no electric field is applied, heat transfer coefficients are influenced by the gravity level, particularly at the low flow rates. On the other hand, in the electrohydrodynamic regimes, heat transfer rates are not only enhanced, but also no longer gravity-dependent, showing that the resulting convection is dominated by the electric field intensity, conveniently controllable by the applied high voltage. Relatively small pressure drop increases caused by the induced flow were also measured. Profitable implementation of electrohydrodynamics in the design of compact heat exchangers and heat sinks such as cold plates is foreseen; possible benefits are pumping power reduction, size and weight reduction, and heat exchange capability augmentation. [ABSTRACT FROM AUTHOR]

The application of a strong electric field within a dielectric medium can produce a jet-like motion of ionized liquid from a high-voltage electrode to a grounded heated plate. Ionic jet impingement on the upper surface of a horizontal plate where boiling is occurring significantly modifies the boiling curve. Heat transfer enhancement has been observed both at saturation temperature and in subcooled conditions, up to a regime of fully-developed nucleate boiling. In addition, the ionic jet is capable of preventing the formation of a vapor film on the boiling surface up to much higher heat fluxes. The working fluids are FC-72 and HFE-7100. Two different high-voltage electrodes have been tested: a single pin and an array of 7 points. This novel electrohydrodynamic technique of ionic-jet impingement boiling can greatly increase, with negligible or very low power input, both the heat transfer coefficients on a heated plate and the critical heat flux for the transition to the film boiling regime (up to 160%). [ABSTRACT FROM AUTHOR]

Laminar to weakly turbulent forced convection in a square duct heated from the bottom is strengthened by ion injection from an array of high-voltage points opposite the heated strip. Both positive and negative ion injection are activated within the working liquid HFE-7100 (C4F9OCH3), with transiting electrical currents on the order of 0.1 mA. Local temperatures on the heated wall are measured by liquid crystal thermography. The tests are conducted in a Reynolds number range from 510 to 12,100. In any case, heat transfer is dramatically augmented, almost independently from the flow rate. The pressure drop increase caused by the electrohydrodynamically induced flow is also measured. A profitable implementation of the technique in the design of heat sinks and heat exchangers is foreseen; possible benefits are pumping power reduction, size reduction, and heat exchange capability augmentation. [ABSTRACT FROM AUTHOR]

The heat transfer characteristics of an electrohydrodynamically induced submerged impinging liquid flow are studied in a point-plane configuration. The working fluid, HFE-7100, is a weakly polar dielectric. Thermofluid-dynamic conditions as close as possible to the microgravity ones are obtained by heating a confined, downward-facing plate. Heat transfer coefficients on the plate can be stably enhanced by over 10 times, with negligible additional work, by means of the ion injection technique. [ABSTRACT FROM AUTHOR]

With the present article, the authors summarize over 15 years of work dedicated to studying the effects of the electrical and the gravitational force fields on two-phase and single-phase thermofluid dynamics. Results obtained on several microgravity platforms are presented and the role played by the electric field in the different heat transfer processes is analyzed. In particular, the regimes of nucleate boiling and film boiling are treated. Also, the parameters controlling the liquid–vapor interface instability and the main consequences regarding the critical heat flux (CHF) are outlined. In the final section, the promising technique of ion injection for efficient heat transfer enhancement in single-phase liquids is described, together with the main results obtained under different flow regimes and geometries. The following dielectrics were compared: R113, Vertrel XF, and FC-72 for the pool boiling research; again FC-72 and HFE-7100 for the single-phase convection heat transfer experiments. [ABSTRACT FROM AUTHOR]

Abstract: Upward flow of perfluorohexane (FC-72) in the entry zone of a uniformly heated circular duct was studied in a regime of transitional combined forced and free convection. Heat transfer coefficients were measured along the heated length at different values of flow rate and heat flux. Heat transfer impairment in the developing region due to laminarisation of the turbulent flow was observed and characterised by means of two well-known dimensionless groups: the Grätz number and the Grashof number based on the wall heat flux. [Copyright &y& Elsevier]

Abstract: The heat transfer characteristics of an electrohydrodynamically induced impinging liquid flow were studied in point-plane geometry. Points of four different metals were tested, varying the high voltage and the point-to-plane spacing. Heat transfer coefficients augmented more than 200%, with a negligible Joule dissipation. The Nusselt number on the stagnation point was successfully correlated to a dimensionless group, behaving similar to the Reynolds number in submerged jets. [Copyright &y& Elsevier]

The thermofluid-dynamic effects of ion injection from sharp metallic points added perpendicularly to the inner wire of a short horizontal annulus were experimentally investigated. A dielectric liquid (FC-72 by 3M) was weakly forced to flow in the duct, which was uniformly heated on the outer wall. A dc voltage as high as 22 kV was applied to the inner electrode, while the heated wall was grounded. Both the laminar and the turbulent mixed-convection regimes were obtained, varying the imposed flow rate. Once an electric field is applied, the flow is dramatically modified by the jets of charged particles, which transfer their momentum to the neutral adjacent ones. Different injection strengths were obtained on the emitters, because the shape of the point tips was not controlled at the microscale. Nusselt number distributions were obtained azimuthally and longitudinally, monitoring the wall temperatures. In all cases, heat transfer turned out greatly enhanced in the proximity of the emitters, without a significant increase in pressure drop through the test section and with a negligible Joule heating, making this technique very attractive for application in compact heat exchangers. [ABSTRACT FROM AUTHOR]

Abstract: In the presence of a sharp HV electrode, free charge can build up in a single-phase liquid by ion injection at the metal/liquid interface. Electrophoretic forces acting on ions can generate strong convective motion, thus augmenting the heat transfer rate. In a vertical annular duct, uniformly heated on the outer wall, a dielectric liquid is weakly forced to flow upward. Sharp points are added perpendicularly to the inner cylinder and d.c. voltages as high as 22 kV are applied to it, while the outer wall is grounded. Prior to the application of the electric field, a regime of turbulent aided mixed convection is obtained. Being in a region of thermally developing flow, laminarization effects are observed, with local heat transfer coefficients depending non-trivially on longitudinal position, heat flux, and flow rate. With the electric field on, heat transfer turns out to be only weakly influenced by the heat flux and the flow rate. The heat exchange reveals to be highly enhanced by this technique and can be modulated by varying the applied voltage. In all cases, the heat transfer improvement is accompanied by a slight increase in pressure drop through the test section and a negligible Joule heating. [Copyright &y& Elsevier]

Abstract: The preliminary experimental results of the effect of an electrostatic field on turbulent aided mixed convection in a short vertical annulus are reported herein. A dielectric liquid (FC-72 by 3M) is used as working fluid. The local heat transfer improvement is obtained by inserting appropriate points on the inner surface of the annulus, generally acting as the positive electrode, while the surrounding pipe is grounded. A high voltage (up to 22 kV) is established between the two surfaces. The heat exchange revealed to be highly enhanced by this technique, thus providing encouraging indications for practical applications. [Copyright &y& Elsevier]

Flow of perfluorohexane in the entry region of a uniformly heated horizontal cylindrical duct was studied in a regime of weakly turbulent mixed convection. Heat transfer coefficients were measured at five cross sections along the heated length and various values of flow rate and heat flux were imposed. A different thermofluid-dynamic behavior was observed between the upper and lower sides of the pipe. Correlations of the Nusselt number for developed and developing flow were proposed, showing agreement with the experimental data within the 10% deviation band. [ABSTRACT FROM AUTHOR]