1,720,991 research outputs found
Viscous dissipation effects on slip flow heat transfer in rhombic microchannels
Full text linkThis study aimed to numerically investigate the viscous dissipation effect on forced convection in rhombic microchannels for gases in a slip flow regime. The numerical analysis was carried out by assuming a 2D steady-state flow. The solution of governing equations was obtained by adopting the finite element method and assuming that the fluid is Newtonian, with constant thermophysical properties, and in a fully developed laminar flow regime. The solution of the momentum equation is obtained by considering a first-order boundary condition, while the solution of the energy balance equation is obtained by assuming a constant wall heat flux (H2 boundary condition) and taking into account the wall temperature jump. The validation of the numerical model was carried out using the data available in the scientific literature. The numerical outcomes obtained for several values of the acute angle of the rhombus, the Knudsen number (i.e., rarefaction effects), and the Brinkman number (i.e., viscous dissipation effects) reveal that viscous forces play an opposite role with respect to rarefaction and significantly affect the convective heat transfer coefficient, especially for low Knudsen numbers and for high values of the acute angle. In particular, it was observed that Nu is significantly affected by the Brinkman number for acute angles higher than 50°
Influence of Thermal Boundary Conditions and Number of Channels on the Performance of Heat Sinks with Rectangular Minichannels
Full text linkThis paper aims to contribute to the analysis of a heat sink designed for the active cooling of small flat surfaces. The heat transfer device investigated here consists of a flat square substrate and a cover, separated by parallel channels with a rectangular cross-section. The cold air flowing in the channels is sucked from the environment, and the bottom of the substrate adheres closely to the hot surface of the device to be cooled. The thermal problem is tackled by considering two different conditions: the first one assuming one long side of the channel is heated and the three other sides are adiabatic (version 1L) and the second one assuming high conductivity of the walls (version 4), in both the H1 and H2 boundary conditions. Moreover, to investigate the effect of the number of channels on the performance of the heat sink, the number of channels is changed between 1 and 20. The results, presented in terms of the f Re product, Nusselt number, maximum surface temperature, and thermal resistance, reveal that both the thermal boundary conditions and the number of channels significantly affect the performance of the investigated heat transfer device
Impact of cross-section geometry on microchannel heat sink performance in the presence of slip and temperature jump boundary conditions
Full text linkThe geometry of microchannels plays a crucial role in determining the heat transfer efficiency and pressure drop in microchannel heat sinks. The choice of channel shape is often guided by practical factors, such as manufacturability, system integration, and cost-effectiveness. This study aims to evaluate the thermohydraulic performance of laminar flow in microchannels with various cross-sectional geometries, including rectangular, trapezoidal, double-trapezoidal, elliptical and rhombic shapes. This analysis considers the effects of slip velocity and temperature jump at the channel walls, as to date, research on the thermal characterization of microchannels operating in the slip flow regime – particularly those with hydrophobic or superhydrophobic surfaces – remains limited. The heat transfer problem is addressed by assuming a constant heat transfer rate along the axial direction of the channel walls with a uniform temperature distribution around the perimeter of the cross-section. This condition is particularly relevant for microchannels subjected to an imposed heat flux, especially when the channel walls are constructed from materials with high thermal conductivity. The numerical results reveal that rectangular and double-trapezoidal (hexagonal) cross-sections demonstrate the highest thermohydraulic performance, whereas the rhombic cross-section performs the worst. Moreover, the outcomes presented here highlight that the rhombic geometry is the most sensitive to slip effects, while the double-trapezoidal geometry is the least sensitive
Space Heating Load Estimation Procedure for CHP Systems sizing
No full textDue to its environmental and energy benefits, the Combined Heat and Power (CHP) represents certainly an important measure to improve energy efficiency of buildings. Since the energy performance of the CHP systems strongly depends on the fraction of the useful cogenerated heat (i.e. the cogenerated heat that is actually used to meet building thermal demand), in building applications of CHP, it is necessary to know the space heating and cooling loads profile to optimise the system efficiency. When the heating load profile is unknown or difficult to calculate with a sufficient accuracy, as may occur for existing buildings, it can be estimated from the cumulated energy uses by adopting the loads estimation procedure (h-LEP). With the aim to evaluate the useful fraction of the cogenerated heat for different operating conditions in terms of buildings characteristics, weather data and system capacity, the h-LEP is here implemented with a single climate variable: the hourly average dry- bulb temperature. The proposed procedure have been validated resorting to the TRNSYS simulation tool. The results, obtained by considering a building for hospital use, reveal that the useful fraction of the cogenerated heat can be estimated with an average accuracy of ± 3%, within the range of operative conditions considered in the present study
Thermal Characterization of Rarefied Flows in Rhombic Microchannels
Full text linkThis work aimed to numerically investigate the dynamic and thermal behavior of a fully developed, laminar, gaseous flow in a microchannel featuring a rhombic cross-section. Due to new fabrication techniques, microducts with rhombic cross-sections have recently received more attention. The momentum and energy balance equations were solved by using a commercial CDF code and assuming the slip and the H2 boundary conditions. The temperature jump between the wall and the adjacent fluid was also taken into account. The accuracy of the numerical results was checked by using the data available in the literature in terms of velocity profiles in the slip flow regime and the Nusselt number in the continuum flow regime. To also investigate the geometry effects on the fluid behavior, several values of the side angle of the rhombus were considered. The numerical results revealed that the rarefaction degree and geometrical properties significantly affected the Nusselt number
Numerical investigation of compressibility effects on friction factor in rectangular microchannels
Full text linkThis study numerically investigated the effect of gas compressibility on the friction factor in rectangular microchannels. The numerical model adopted in the study was validated against experimental data obtained by testing rectangular microchannels with a hydraulic diameter of 295 μm. The numerical model was used to evaluate the Reynolds number at which the compressibility effects on the friction factor became significant by analyzing the role of both hydraulic diameter and aspect ratio of the microchannel. To this end, three values of the hydraulic diameter (100, 295, and 500 μm) and five different aspect ratios (from 0.25 to 1) were investigated. The results showed that compressibility effects became increasingly stronger by reducing the hydraulic diameter and that they led to an increase in the average friction factor. For smaller microchannels, the Reynolds numbers at which the compressibility effects became significant tended to reduce and were in the laminar regime. The gas compressibility could not be ignored when friction factors needed to be accurately determined. Moreover, for narrow microchannels (low aspect ratio), compressibility effects became important for higher values of the Reynolds number than those observed for nearly squared microchannels
Toward Improved Urban Building Energy Modeling Using a Place-Based Approach
No full textUrban building energy models present a valuable tool for promoting energy efficiency in building design and control, as well as for managing urban energy systems. However, the current models often overlook the importance of site-specific characteristics, as well as the spatial attributes and variations within a specific area of a city. This methodological paper moves beyond state-of-the-art urban building energy modeling and urban-scale energy models by incorporating an improved place-based approach to address this research gap. This approach allows for a more in-depth understanding of the interactions behind spatial patterns and an increase in the number and quality of energy-related variables. The paper outlines a detailed description of the steps required to create urban energy models and presents sample application results for each model. The pre-modeling phase is highlighted as a critical step in which the geo-database used to create the models is collected, corrected, and integrated. We also discuss the use of spatial auto-correlation within the geo-database, which introduces new spatial-temporal relationships that describe the territorial clusters of complex urban environment systems. This study identifies and redefines three primary types of urban energy modeling, including process-driven, data-driven, and hybrid models, in the context of place-based approaches. The challenges associated with each type are highlighted, with emphasis on data requirements and availability concerns. The study concludes that a place-based approach is crucial to achieving energy self-sufficiency in districts or cities in urban-scale building energy-modeling studies
Outdoor temperature sensitivity of electricity consumption for space heating and cooling: An application to the city of Milan, North of Italy
No full textWith the aim of investigating the effect of climate on the electricity use for space heating and cooling, the correlation between the daily electricity consumption of the city of Milan (Italy) in the five-year period 2013–2017 and the daily average outdoor dry bulb temperature of the same location was analyzed by means of statistical tools. To filter out the effect of the variables different from weather the analysis was limited to workdays. Electricity use and outdoor temperature were correlated by using a parametric model, within a parameter estimation approach, in order to highlight the relevant physical phenomena and to identify the value of the building stock characteristic parameters. A modified five-parameter model (M5PM) was proposed, based on the second principle of thermodynamics, which accounts for the effect of the past temperature by adopting an effective temperature approach. The comparison between the actual data for the workdays of the whole 5-year period and the electricity consumption obtained by means of the M5PM revealed a good agreement between the two distributions, as confirmed by the value of the coefficient of determination (R2 = 0.93), by the value of the normalized root mean square error (NRMSE = 2.3%) and by the value of the mean absolute percentage error (MAPE = 1.3%). The regression model was then applied to the electricity use analysis of the individual days of the week. By increasing the number of data set and their disaggregation, the approach based on regressing the energy consumption historical data series could be successfully adopted for applications that are currently approached by direct simulation or by measurement. In this perspective big data analytics in combination with the parameter estimation approach remains a promising tool to facilitate the interpretability of the energy use model
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