Cooling Air Flow Rate Research Articles

Performance of convective-radiative porous fin heat sink under the influence of particle deposition and adhesion for thermal enhancement of electronic components

This work presents an investigation on the effect of particle deposition and adhesion on the thermal behaviour of convective-radiative porous fin heat sink using Differential Transform method (DTM). The numerical solutions of the developed thermal models are also used to study the influence of thermal conductivity, porosity, convection and radiation factors on the thermal distribution and efficiency of the fin. The analysis reveals the impact of particles or thermal fouling on the surface of the fin, which causes a rise in the temperature uniformity and distribution in the fin while reducing the rate of heat transfer from the fin. Moreover, the study also establishes that the heat transfer rate decreases due to particle deposition on the fin which depends on the particle size, particle density and cooling air flow rate. The fin efficiency decreases with increasing value of the fouled Biot, Darcy and radiation numbers, and thermo-geometric parameter. Furthermore, the analysis shows that the efficiency of the fouled porous fin is always greater than the efficiency of the clean porous fin. The result of this study shows an excellent agreement with the established results of Runge-Kutta with shooting method.

Engine-Control Impact on Energy Balances for Two-Stroke Engines for 10–25 kg Remotely Piloted Aircraft

The rapid expansion of the market for remotely piloted aircraft (RPA) includes a particular interest in 10–25 kg vehicles for monitoring, surveillance, and reconnaissance. Power-plant options for these aircraft are often 10–100 cm3 internal combustion engines (ICEs). The present study builds on a previous study of loss pathways for small, two-stroke engines by quantifying the trade space among energy pathways, combustion stability, and engine controls. The same energy pathways are considered in both studies—brake power, heat transfer from the cylinder, short circuiting, sensible exhaust enthalpy, and incomplete combustion. The engine controls considered in the present study are speed, equivalence ratio, combustion phasing (ignition timing), cooling-air flow rate, and throttle. Several options are identified for improving commercial-off-the-shelf (COTS)-engine efficiency and performance for small, RPA. Shifting from typical operation at an equivalence ratio of 1.1–1.2 to lean operation at an equivalence ratio of 0.8–0.9 results in a 4% (absolute) increase in fuel-conversion efficiency at the expense of a 10% decrease in power. The stock, linear timing maps are excessively retarded below 3000 rpm, and replacing them with custom spark timing improves ease of engine start. Finally, in comparison with conventional-size engines, the fuel-conversion efficiency of the small, two-stroke ICEs improves at throttled conditions by as much as 4–6% (absolute) due primarily to decreased short-circuiting. When no additional short-circuiting mitigation techniques are employed, running a larger engine at partial throttle may lead to an overall weight savings on longer missions. A case study shows that at 6000 rpm, the 3W-55i engine at partial throttle will yield an overall weight saving compared to the 3W-28i engine at wide-open throttle (WOT) for missions exceeding 2.5 h (at a savings of ∼5 g/min).

Effect of occupant behavior and air-conditioner controls on humidity in typical and high-efficiency homes

Increasing insulation levels and improved windows are reducing sensible cooling loads in high-efficiency homes. This trend raises concerns that the resulting shift in the balance of sensible and latent cooling loads may result in higher indoor humidity, occupant discomfort, and stunted adoption of high-efficiency homes. This study utilizes established moisture-buffering and air-conditioner latent degradation models in conjunction with an approach to stochastically model internal gains. Building loads and indoor humidity levels are compared for simulations of typical new construction homes and high-efficiency homes in 10 US cities. The sensitivity of indoor humidity to changes in cooling set point, air-conditioner capacity, and blower control parameters are evaluated. The results show that high-efficiency homes in humid climates have cooling loads with a higher fraction of latent loads than the typical new construction home, resulting in higher indoor humidity. Reducing the cooling set point is the easiest method to reduce indoor humidity, but it is not energy efficient, and overcooling may lead to occupant discomfort. Eliminating the blower operation at the end of cooling cycles and reducing the cooling airflow rate also reduce indoor humidity and with a smaller impact on energy use and comfort.

A Lumped Thermodynamic Model of Gas Turbine Blade Cooling: Prediction of First-Stage Blades Temperature and Cooling Flow Rates

Turbine inlet temperatures (TIT) of 1500–2000 K have become a sort of standard for most modern advanced applications. First-stage blades are obviously the most exposed components to such hot gases, and thus they need proper cooling. In the preliminary design of the blades and their cooling system, designers must rely on simple models that can be further refined at a later stage, in order to have an approximate but valuable set of guidelines and to reach a feasible first-order configuration. In this paper, a simple lumped thermodynamic model of blade cooling is proposed. It is based on mass/energy balances and heat transfer correlations, and it predicts a one-dimensional temperature profile on the blade external surface along the chord for a given gas temperature profile, as well as the required cooling air flow rates to prevent blade material from creep. The greatest advantage of the model is that it can be easily adapted to any operating condition, process parameter, and blade geometry, which makes it well suited to the last technological trends, namely, the investigation of new cooling methods and alternative coolants instead of air. Therefore, the proposed model is expected to be a useful tool in the field of innovative gas turbine cycle analysis, replacing more computationally intensive and very time-consuming models.

Effects of particles deposition on thermal performance of a convective-radiative heat sink porous fin of an electronic component

The advent of high-power electronic system components and slim margin errors require the proper understanding of particle deposition characteristics and its effects on the thermal performance of electronic heat sinks of electronic devices. In this work, we investigate the effects of particle deposition on the thermal performance of convective-radiative porous fin heat sink of an electronic component using Chebyshev spectral collocation method. The numerical solutions from the developed thermal models are also used to study the influence of thermal conductivity, porosity, convection, and radiation parameter on the temperature distribution and overall fin efficiency. The analysis establishes that presence of particles or thermal fouling on the surface of the fin increases the temperature distribution and temperature uniformity of the fin whilst decreasing the heat transfer rate from the fin. The study also establishes that decrease in heat transfer rate from the fin is due to the particle deposition on the fin, which depends on the particle size, particle density and cooling air flow rate. Moreover, the fin efficiency decreases with an increasing value of the fouled Biot, Darcy and radiation numbers, and thermo-geometric parameter. The study further shows that the efficiency of the fouled porous fin is always greater than the efficiency of the clean porous fin. The analysis established that the result of Chebyshev spectral collocation method is in good agreement with that of the convectional numerical method using Runge-Kutta coupled with shooting method.

An experimental study of flow and heat transfer performance of a longitudinal corrugated liner for a combustion chamber

Experiments are conducted to investigate the flow and heat transfer performance of a longitudinal corrugated liner for a combustion chamber. The effect of cooling air passage height and blockage ratio on the heat transfer performance of the liner has been studied. In addition, the friction factors in cooling air passages are also presented and the test results are compared with previously reported studies. In the experiments, the pressure fields having the same conditions as that present in the real engine are established. The detailed temperature distributions on the hot side of the liner are measured by using the infrared imaging technique. Local film cooling effects are calculated from these temperature distributions. A range of parameter combinations of interest in cooled liner practice is covered, including combinations of variations in Reynolds number of cooling air passage, passage height and blockage ratio. The results indicate that the blockages in the cooling air passage are not beneficial for obtaining a high film cooling effect and for the uniform distribution of film cooling effect in the streamwise direction. The cooling air flow will be disturbed by the blockages located in the cooling air passage which weakens the effect of dynamic pressure on cooling air injection. The local and downstream film cooling effect will be significantly influenced. The friction factor increases with the increase of bolckage ratio. Compared with previous works, the friction factors in the present conditions are much higher (at least by 133%) than that for parallel plat passage and pipes with rough walls. As the passage height increases, the film cooling effect increases first and then decreases. However, the relative deviation decreases first and then increases. That means there is an optimum passage height for a higher film cooling effect and a uniform distribution. The passage height has an significant effect on the momentum of cooling air injection and the distribution of cooling air flow rate through the windward side of the crest and the leeward side of the crest. The effect of passage height on the friction factor is slight. The f-Re correlations are proposed based on the present experimental data.

Numerical Simulation on Flow and Heat Transfer Performance of Air-cooler for a Natural Gas Storage Compressor Unit

Heat transfer efficiency has been a key issue for large size air coolers with the noise reducers used in natural gas storage compressor unit, especially operated in summer with cooling air at a high temperature. The 3-D numerical simulation model of the whole air cooler was established to study the flow field characteristic with different inlet and outlet structures by CFD software. The system pressure loss distributions were calculated. The relationship was obtained among heat exchange efficiency, resistance loss, and the structure of air cooler, the results presented some methods to improve cooling air flow rate and heat exchange efficiency. Based on the results, some effective measures were proposed to improve heat exchanger efficiency and were implemented in the actual operation unit.

Developing heat source term including heat generation at rest condition for Lithium-ion battery pack by up scaling information from cell scale

Heat generation during cycling of Lithium-ion batteries may have negative effects on the efficiency, safety, performance and lifetime of battery cell due to overheating. Therefore, temperature distribution should be controlled by the battery thermal management techniques. To improve accuracy of temperature control methods, the value of heat generation should precisely be calculated. However, detail modeling of phenomena, inside all cells of a battery pack, is a time consuming process, impracticable for control purposes. The prime objective of this paper is to propose a developed analysis viewpoint of heat generation phenomenon at pack scale by presenting a relation derived from the cell scale analysis without missing the accuracy along with vanishing computational cost. In order to increase the accuracy, firstly, the thermal modeling part at cell scale is improved to consider the heat of mixing and enthalpy heating terms as significant heat sources continue to be generated after cutoff current. Then, based on output data of the corrected code, a relation for heat generation source term is estimated by neural network method. Finally, this relation is used as a source term in pack scale equations. The results demonstrated the importance of considering heat of mixing and enthalpy heating in the pack scale modeling in a wide range of cooling air flow rates. This method is generic and vanishes the computational cost of transferring information from cell scale to the pack one.

Study on solid liquid interface heat transfer of PCM under simultaneous charging and discharging (SCD) in horizontal cylinder annulus

Heat transfer performance during the simultaneous charging and discharging (SCD) operation process for phase change materials (PCM) contained inside the annulus of concentric horizontal cylinder was investigated. In the experimental set-up, the PCM inside the annulus serves as the heat sink along with an externally imposed forced cooling air. The obtained time wise temperature profile was used to determine the effects of different heat fluxes and the imposed forced convection cooling on the melt fraction values and the transition shift time from the observed conduction to natural convection heat transfer patterns. Furthermore, non-dimensional analysis was presented for the heat transfer at the interface to enable generalizing the result. Comparison of the results show that the SCD operation mode establish the condition that enables much PCM phase transition time and thus longer time of large latent heat transfer effect than the Partial and non simultaneous operations. Analysis results show that the variation of the heat flux for the SCD mode did not change the dominance of the natural convection over conduction heat transfers in the PCM. However, it significantly influences the commencement/transition shift time and melting rate while higher heat fluxes yields melt fraction that was 38–63% more for investigated process time. Variation with different cooling air flow rate shows more influences on the melt fraction than on the mode of heat transfer occurring in the PCM during melting. Available non-SCD modes correlation was shown to be insufficient to accurately predict interface heat transfer for the SCD modes.

Varying duty operation of air-cooled condenser units

Results of experimental investigations of operation modes of air-cooled condensers (ACC) under design and varying duty conditions are presented. ACCs with varying cooling airflow rates under constant heat load and with constant cooling airflow under varying heat load are examined. Diagrams of heat transfer coefficients and condensation pressures on the heat load and cooling airflow are obtained. It is found that, if the relative heat load is in the range from 0.6 to 1.0 of the nominal value, the ACC heat transfer coefficient varies insignificantly, unlike that of the water-cooled surface condensers. The results of the determination of “zero points” are given, i.e., the attainable pressure in air-cooled condensing units (ACCU), if there is no heat load for several values of working water temperature at the input of water-jet ejectors and liquid ring vacuum pump. The results of the experimental determination of atmospheric air suction into the ACC vacuum system. The effect of additional air suctions in the steam pipe on ACCU characteristics is analyzed. The thermal mapping of ACC heat exchange surfaces from the cooling air inlet is carried out. The dependence of the inefficient heat exchange zone on the additional air suction into the ACC vacuum system is given. It is shown that, if there is no additional air suction into the ACC vacuum system, the inefficient heat exchange zone is not located at the bottom of the first pass tubes, and their portion adjacent to the bottom steam pipe works efficiently. Design procedures for the ACC varying duty of capacitors are presented, and their adequacy for the ACCU varying duty estimation is analyzed.

Thermotechnical performance of an air-cooled tuyere with air cooling channels in series

To reduce the cooling air consumption for an air-cooled tuyere, an air-cooled tuyere with air cooling channels in series is developed based on several hypotheses, i.e., a transparent medium in the blast furnace, among others, and the related mathematical models are introduced and developed. Referring to the data from a BF site, the thermotechnical computation for the air-cooled tuyere was performed, and the results show that when the temperature of the inlet cooling air increases, the temperatures for the outlet cooling air, the outer surface of the tuyere, the walls of the air cooling channels and the center channel as well as the heat going into the center channel increase, but the heat absorbed by the cooling air flowing through the air cooling channels decreases. When the cooling air flow rate under the standard state increases, the physical parameters mentioned above change in an opposite directions. Compared to a water-cooled tuyere, the energy savings for an air-cooled tuyere are more than 0.23 kg/min standard coal.

Thermal performance of a PCB channel heat sink for LED light bulbs

In this study, the thermal performance of the printed circuit board (PCB) as a heat sink for the light-emitting diode (LED) bulb is analyzed. Cooling air is drawn through the bottom inlet of a case and exchanges heat from a vertically aligned PCB acting as a heat sink fin. The air then exits through a top outlet. Cooling performance is improved by arranging parallel PCBs to form a channel. Heat transfer is optimized by balancing cooling airflow rates on external and internal PCB surfaces, thereby reducing thermal resistance by 30% compared to previous designs. Cooling performance according to installation angle is investigated. The orientation effect on this channel design is determined to be 10% less than that of the reference design. Finally, the effect of PCB channel cooling performance on the LED bulb’s life span is analyzed and shown to be 40% longer than that of the existing geometry.

Air-cooled gas turbine cycles – Part 1: An analytical method for the preliminary assessment of blade cooling flow rates

It is well known that, for a given compressor technology, gas turbine efficiency increases with the turbine inlet temperature (TIT): both modern aeronautical and land-based gas turbines operate at very high temperatures (1500–2000K) –and correspondingly high pressure ratios. As the TIT increases, the heat transferred from the expanding gas to the turbine blade also increases, and the need to extend the operational life make it necessary to adopt internal air cooling to reduce blade creep, oxidation and low-cycle fatigue. The cooling medium is usually air extracted from the high-pressure compressor stages, and since this extraction decreases the thermal efficiency and power output of the engine, it is important to bleed the minimum amount of coolant to attain a prescribed maximum material temperature in the blade with the maximum possible uniformity (lower thermal stresses): thence the need to properly model the cooling system for a given turbine blade geometry under realistic engine operating conditions. In the preliminary design of the first statoric and rotoric blading, it is essential for designers to rely on simple models that often neglect the small scales effects on the external flows and also by force adopt a much simplified treatment of the internal ones, and as a result attain a substantially lower degree of approximation than that offered by more complex and expensive numerical simulations. The goal in the design of a lumped model is therefore to make it both sufficiently general and accurate to analyze blade shapes and cooling channels structures that can be further refined by means of more accurate, but also more computationally intensive, models.This paper presents a simple, globally lumped thermodynamic model of blade cooling whose most important feature is its being analytical, so that the solution is devoid of numerical approximations and leads to closed-form expressions that can be easily manipulated to accommodate for different process parameters, blade arrangements, cooling channel structure and fluid properties. The model predicts the mass-averaged gas temperature along the channel and the required cooling air flowrate, under the assumption of a constant metal temperature along and inside of the blade: therefore, thermo-mechanical effects are completely neglected.The results are validated against some published data, and the agreement is found to be quite satisfactory: when discrepancies arise, a phenomenological explanation is offered.

Photovoltaic-thermal (PV/t) panel to minimize electrical and air conditioning energy consumption of a typical office in Beirut

ABSTRACTA combined photovoltaic–thermal (PV/t) panel is proposed to produce simultaneously electricity and heat from one integrated unit. The unit utilizes effectively the solar energy through achieving higher PV electrical efficiency and using the thermal energy for heating applications. To predict the performance of the PV/t at a given environmental conditions, a transient mathematical model was developed. The model was integrated in a heating application for a typical office space in the city of Beirut to provide the office needs for electricity, heating during winter season, and dehumidification and evaporative cooling during the summer season. To minimize the yearly office energy (electrical and heat) needs, the PV/t panel cooling air flow rate and the dehumidification regeneration temperature were determined for opimal unit operation. Thermal energy savings of up to 85% in winter and 71% in summer were achived compared to conventional systems at a payback period of 8 years for the panels.

Study of Influence Effect on Heat Transfer Performance of Single-Loop Oscillating Heat Pipe

s. The experiment studied the effect of heat source temperature, heating section length ratio, cooling air flow rate, liquid filling rate and pipe diameter on the heat transfer performance of the single-loop red copper-water oscillating heat pipe. The results show that increasing heat source temperature or pipe diameter and reducing filling rate can obviously reduce the thermal resistance of the heat pipe; in the air cooling mode, the cooling thermal resistance outside the pipe is affected by both cooling conditions and heat pipe cooling section average temperature; when the heating section is shorter than the cooling section, the heat pipe thermal resistance shows an apparent trend of increasing with the increase of heating section length ratio, when the heating section is longer than the cooling section, the cooling thermal resistance increases with it apparently; the heat transfer power is the highest when the filling rate is 50%.

Energy and exergy analysis for waste heat cascade utilization in sinter cooling bed

In the present paper, a numerical study is presented to investigate the cascade utilization of waste heat in sinter cooling bed. With the aid of CFD (computational fluid dynamics), a two-dimensional unsteady mathematical model, which would significantly reduce the computational time, is established to describe three-dimensional steady flow and heat transfer in sinter cooling bed. The Brinkman–Forchheimer extended Darcy model and the LTNE (local thermal non-equilibrium) model are employed to describe flow and heat transfer in sinter cooling bed. And the reliability of this mathematical model is validated with both related simulation and experimental work. And then, numerical simulations are conducted to examine the effects of different operating parameters on the cooling air temperature and waste heat utilization quantity. Furthermore, the waste heat grade and quantity are taken into comprehensive consideration in energy and exergy analysis. The results indicate that, both the quantity and quality of waste heat utilization would be improved by increasing sinter cooling bed height, trolly's moving speed and sinter heat flux. Meanwhile, it is also found that, with different assignments of cooling air flow rate, the quantity and quality of waste heat in sinter cooling bed would not be improved at the same time.

Steady State Model of Cooling Tower Used in Air Conditioning

To predict the performance of cooling tower used in air conditionings in variable operating conditions, a steady state model of cooling tower suitable for variable conditions simulation was built and tested. Comparison between the prediction value and the experimental value shows good agreement. The results show that this model is suitable for calculation of cooling capacity of cooling tower used in air conditionings at variable inlet temperature or mass flow rate of condensing water, and at variable inlet wet-bulb temperature or mass flow rate of cooling-air. This model is useful for simplifying performance test, equipment selection and running control.

Analysis of Influencing Factors of Pressure Pre-Cooling Rate for Fruits and Vegetables

Cooling rate is an important evaluation index of pressure pre-cooling effect for fruits and vegetables. Experimental device of pressure pre-cooling for fruits and vegetables has been established. Pre-cooling process of golden pears has been tested. The key parameters which affected pressure pre-cooling 7/8 cooling time of golden pears such as different air flow rates, different shapes and sizes of vent hole and arrange form have been analyzed. The results show that it is better that cooling air flow rate is between 1.5 m/s and 2 m/s. Ellipse vent hole shape is the best vent hole style and key-groove vent hole is the worst. The cooling rate of stagger array form is faster than the parallel array form.

Experimental Feasibility Study of Radial Injection Cooling of Three-Pad Air Foil Bearings

The foil bearing (FB) is one type of hydrodynamic bearing using air or another gas as a lubricant. When FBs are designed, installed, and operated properly, they are a very cost-effective and reliable solution for oil-free turbomachinery. Because there is no mechanical contact between the rotor and its bearings, quiet operation with very low friction is possible once the rotor lifts off the bearings. However, because of the high speed of operation, thermal management is a very important design factor to consider. The most widely accepted cooling method for FBs is axial flow cooling, which uses cooling air or gas passing through heat-exchange channels formed underneath the top foil. The advantage of axial cooling is that no hardware modification is necessary to implement it, because the elastic foundation structures of the FB serve as the heat-exchange channels. Its disadvantage is that an axial temperature gradient exists on the journal shaft and bearing. In this paper, the cooling characteristics of axial cooling are compared with those of multipoint radial injection, which uses high-speed injection of cooling air onto the shaft at multiple locations. Experiments were performed on a three-pad FB 49 mm in diameter and 37.5 mm in length, at speeds of 30,000 rpm and 40,000 rpm. Injection speeds were chosen to be higher than the journal surface speed, but the total cooling air flow rate was matched to that of the axial cooling cases. Experimental results show that radial injection cooling is comparable to axial cooling at 30,000 rpm, in terms of cooling performance. Tests at 40,000 rpm reveal that the axial cooling performance reaches saturation when the pressure drop across the bearing is larger than 1000 Pa, while the cooling performance of radial injection is proportional to the cooling air flow rate and does not become saturated. Overall, multipoint radial injection is better than axial cooling at high rotor speeds.

An experimental study on the performance of a condensing tumbler dryer with an air-to-air heat exchanger

The performance of energy consumption in the closed-loop tumbler dryer with a condenser for clothes drying is evaluated as a function of the heater capacity, the drying air flow rate inside the dryer, and the cooling air flow rate. The clothes dryer in laundries used in this study consists of a tumbling drum, condenser for condensing the humid and hot air flowing out the rotating drums, and electric heater for heating the circulating drying air. Tests were performed at the heating capacity of 1.9 kW to 2.7 kW, the drying air flow rate of 60 m3/h to 140 m3/h, and the cooling air flow rate of 100 m3/h to 240 m3/h. The total energy consumption, the drying time, and the condensate water rate were also investigated. Parametric results showed that a larger heater power resulted in shorter drying time. With increasing heater power, the air temperature and the condensate rate increased due to the higher humidity ratio in the air. The drying air flow rate and the cooling air flow rate did not have a significant effect on drying performance.