Heat Recirculation Research Articles

Numerical study of flame stability within inert porous media with variable void area

This numerical study brings a comparison of a cylindrical porous burner with graded porosity and a conical porous burner with uniform porosity. Both burners have the same void area, creating a similar hydrodynamic stabilization mechanism. The study was performed for a laminar premixed CH4/air flame for different equivalence ratios with the FGM reduction technique employed to model the chemical kinetics. The results demonstrate that the cylinder with graded porosity presents a wider range of stable flames compared with the conical porous burner because the graded porosity leads to a larger heat recirculation as the flame approaches the outlet surface, which is mainly due to an increase of the solid-phase radiative heat transfer. The two-dimensional structure of the flame front was investigated in detail showing that the non-alignment of flow streamlines and solid-phase heat flux lines creates regions of sub-adiabatic and super-adiabatic temperatures along the flame front. Flame area increases with the inlet flow rate with the graded porosity burner presenting a smaller area and larger local consumption velocity than conical burner.

Investigation on the H2 fueled combustion with CH4 and C3H8 blending in a micro tube with/without fins

Combustion characteristics and working performance of the blending hydrocarbon fuels in a micro combustor are investigated. Experiments and simulations are conducted to analyze the effects of CH4 and C3H8 blending on the premixed H2/air burning process and wall temperature distribution. To enhance the radiation power and energy conversion rate of the micro thermophotovoltaic (TPV) system, the combustors inserted with varied fins are proposed and compared. The results indicate that the premixed H2/air combustion with 7.5 % CH4 addition gains the best thermal ability due to the flame location modification and reaction zone extension. Nevertheless, the combustion with 5 % C3H8 blending achieves the highest wall temperature in C3H8/H2/air burning. Moreover, the CH4/H2/air combustion gains a better thermal performance than that of C3H8/H2/air burning under the same blending ratio owing to the descending heat loss from the outlet and enhanced reaction intensity. The reaction pathway and sensitivity analysis demonstrate that the chemical reaction rate related to the OH radical is reduced with the CH4 or C3H8 blending ratio, leading to the stretched flame. The reaction rate of CH4/H2/air combustion is higher than that of C3H8/H2/air, increasing the heat release. Besides, the inserted fins optimize the fluid field and enhance the heat recirculation, then a higher radiation temperature is obtained. The combustor with appropriate fins number and length effectively achieves a satisfactory working performance. In particular, the highest radiation efficiency of 54.0 % and highest radiation power of 69.1 W are obtained in the combustor C8-24 burning with 7.5 % CH4 under chemical energy Ec = 91.5 W and Ec = 141.4 W, respectively.

Analysis of Thermal Emissions of Exoplanets with Axially Symmetric Temperature Gradients

Here a new method of modeling the thermal emissions of exoplanets is described, in which the temperature gradient of an exoplanet is approximated by splitting it into N zones. First, we seek to determine how much this method differs from a simple dayside–nightside model used by previous researchers and found that the difference between the N-zone and the dayside–nightside models is greatest during the primary transit of the exoplanet, and for large temperature gradients. Next, we determine under what conditions EXONEST, a Bayesian inference software package, is able to correctly determine the model used to generate synthetic light-curve data. EXONEST is best able to determine the model used to generate synthetic data when the mass of the exoplanet is known, the added noise to the data is low, and the thermal emissions are large compared to the ellipsoidal variations. Finally, EXONEST was used to analyze photometric data for exoplanets Kepler-41b and Kepler-412b, and the dayside brightness temperatures were estimated to be 2574 ± 59 and 2496 ± 64 K, and those of the nightside were estimated to be 860 ± 316 and 874 ± 333 K for Kepler-41b and Kepler-412b, respectively. Finally, we found that the hottest zone for both planets was the zone nearest the terminator on the dayside of the exoplanet. This surprising result suggests that the model is better applied to exoplanets with little to no heat recirculation.

Characteristics of hollow bluff body-stabilized natural gas-air premixed flames with heat recirculation

The combined effects of flow recirculation and preheating on the premixed flame stability limits were employed via two flow configurations. In the first configuration, the natural gas-air mixture was set to flow around a bluff body whose orientation, number of vertices and vertex angle were varied. In the second configuration, an innovative design was developed where the fuel–air mixture was set to flow inside the bluff body as two opposing jets whose resultant flow is redirected to pass around the bluff body. The peak turbulent kinetic energy (T. K. E.) decreased from 7.24 to 4.94 m2/s2 with the star shape of 4 vertices as its orientation changed from 45 to 0° and such decrease was limited to 5.27 m2/s2 upon having a smaller vertex angle. A smaller drop from 7.24 to 6.0 m2/s2 was experienced when the 4 vertices were replaced by 5 vertices at 0° angle of orientation, while the drop continued to 4.3 m2/s2 upon having 6 vertices due to the reduction in the space available for eddies. A smaller peak T. K. E. of 2.33 m2/s2 was found with the star shape of 3 vertices because the sites for turbulence development were limited. With the star cross-section of 4 vertices at 45° angle of orientation, a maximum blow-off velocity as high as 51.5 m/s and a minimum lean limit as low as 0.40 were obtained. A further extension in the flame stability limits was provided via the second flow configuration due to the favorable effects of impingement and prolonged path of preheating. As the distance between the bluff body and the combustor separating wall decreased to 1.7 times the jet diameter, the blow-off velocity increased to 79.1 m/s; while the lean limit decreased to 0.31 with the star shape of 4 vertices at 45° angle of orientation.

Scale-up of microwave-assisted, continuous flow, liquid phase reactors: Application to 5-Hydroxymethylfurfural production

Microwave (MW) technology can be powerful for the electrification and intensification of chemical manufacturing. However, very few examples of scaled MW processes exist. In this work, we build a scaled, MW-assisted, continuous flow reactor for the processing of liquid phase chemistries with specific application to 5-hydroxymethylfurfural (HMF) production. We demonstrate a corresponding computational fluid dynamics model to simulate the reactor’s temperature profile and performance, both of which are in good agreement with experiments. We construct a surrogate model to relate operating parameters to performance for guiding experiments, without demanding simulations, via active learning. Heat recirculation is demonstrated in this scaled reactor, further extending its scale and reducing the energy demand. An HMF yield of ∼55 % and a productivity of 0.1 kg/hr, 8x higher than any other reactor, at a flowrate >20x than prior work, are demonstrated while maintaining energy efficiency of >98 %. A basic economic analysis estimates a $1.85/kg cost of HMF from fructose and >60 % reduction in CO2 emissions than a conventional system without considering the impact of green electricity. We demonstrate that MW technology is well-applied to systems larger than the laboratory scale, and strategies to further reduction in cost and CO2 are discussed.

Revisiting the Iconic Spitzer Phase Curve of 55 Cancri e: Hotter Dayside, Cooler Nightside, and Smaller Phase Offset

Thermal phase curves of short-period exoplanets provide the best constraints on the atmospheric dynamics and heat transport in their atmospheres. The published Spitzer Space Telescope phase curve of 55 Cancri e, an ultra-short-period super-Earth, exhibits a large phase offset suggesting significant eastward heat recirculation, unexpected on such a hot planet. We present our rereduction and analysis of these iconic observations using the open source and modular Spitzer Phase Curve Analysis pipeline. In particular, we attempt to reproduce the published analysis using the same instrument detrending scheme as the original authors. We retrieve the dayside temperature ( K), nightside temperature ( K at 2σ), and longitudinal offset of the planet's hot spot, and quantify how they depend on the reduction and detrending. Our reanalysis suggests that 55 Cancri e has a negligible hot spot offset of degrees east. The small phase offset and cool nightside are consistent with the poor heat transport expected on ultra-short-period planets. The high dayside 4.5 μm brightness temperature is qualitatively consistent with SiO emission from an inverted rock vapor atmosphere.

Ultra-lean dynamics of holder-stabilized hydrogen-enriched flames in a preheated mesoscale combustor near the laminar critical limit

This work experimentally investigates the ultra-lean dynamics of a 40% H2–60% CH4 flame near the laminar critical limit in a preheated mesoscale combustor with a flame holder. These experiments are conducted to verify a conjecture we proposed in a previous publication and reveal the ultra-lean flame dynamics under the synergistic effects of heat and flow recirculation. Notably, not only is our conjecture confirmed, but also some novel flame behaviors are found. As the equivalence ratio ϕ is decreased from 0.500 to 0.320, the conventional stable flame, stable residual flame, periodic residual flame with repetitive local extinction and re-ignition (periodic RFRER), and periodic oscillating residual flame are observed in sequence. For the stable residual flame (0.370 ≥ ϕ ≥ 0.355), the left and right flame roots reside directly behind the flame holder, and the flame tip stays near the combustor exit. For the periodic RFRER (0.350 ≥ ϕ ≥ 0.340), observed experimentally for the first time, the flame roots reside at almost the same location, but the flame tip oscillates up and down over time with pinch-off events. For the periodic oscillating residual flame (0.335 ≥ ϕ ≥ 0.320), found for the first time, the stable flame roots also reside at almost the same location, but the residual flame tip oscillates up and down over time without a pinch-off event. When ϕ decreases to 0.315, the oscillating residual flame extinguishes, and its blow-off dynamics are revealed in detail.

Evaluation of Radiative Heat-Transfer Effect on Paraffin-Wax Regression Rate in Hybrid Rocket

A numerical setup has been developed to rebuild the axial profile and time history of the regression rate of a paraffin-wax-based fuel in a hybrid rocket, with the main target of investigating the effect of the local radiative heat transfer from both the combustion gas molecules and soot particles. The numerical approach is based on a quasi-one-dimensional computational fluid dynamic strategy, which is coupled with chemical equilibrium combustion; the vaporization and entrainment components of the wax-fuel regression rate have been estimated with an established liquefying-fuel model. The increase of the convective heat-transfer blocking due to radiation and the effect of recirculation flow due to oxidizer injection have been introduced. Numerical results are compared with experimental data. When radiative heat transfer, blocking-effect correction, and recirculation flow effect are taken into account, the calculated and measured fuel regression rates agree well in both values and axial profiles.

Numerical study on the extinction dynamics of partially premixed meso-scale methane-air jet flames with hydrogen addition

In this paper, a model of partially premixed jet flames that sustained above a meso-scale short tube was established for an individual flame port of domestic gas stoves. The effects of hydrogen addition (volume ratio β = 0%, 10%, 20%, 30%) on the extinction dynamics of CH4-air jet flames were numerically investigated. It is found that flame oscillation occurs once (β = 10% and 20%) or twice (β = 30%) in the extinction process. Moreover, the larger of β, the longer the extinction process can sustain. Analysis was performed in terms of both chemical effect and thermal effect. As to the chemical effect, in the first place, the reaction rate decreases as the inlet velocity is reduced. As a result, the consumption rate of O2 will be less than the supply rate from the incoming mixture, which makes the O2 concentration in the flame center increase. On the other hand, the amount of H radicals increases with the increase of β, and when the O2 content at the flame center reaches a “critical point”, the key elementary reaction “H + O2 ↔ O + OH” will be enhanced and consequently the total reaction rate will also be intensified. After that, the consumption rate of O2 will be larger than the supply rate due to the reduced flow rate of incoming mixture. The total heat release rate will decrease sharply and extinction occurs. As regards the thermal effect, it is revealed that heat recirculation effect (indirect preheating effect) lags behind the variation of the reaction zone (i.e., flame), thus, it has a negligible impact on flame oscillation. In contrast, the preheating temperature in the vicinity of flame front (named as “direct preheating effect”) exhibits a similar variation tendency with the total heat release rate of the flame. And the larger of β, the more remarkable of the direct preheating effect can be. In summary, due to the chemical effect and thermal effect caused by hydrogen addition, the flame can survive for a longer time with fluctuation during the extinction process.

Combustion of lean ammonia-hydrogen fuel blends in a porous media burner

Ammonia offers a carbon-free fuel alternative for industrial heating, power generation and transportation. However, its low flame speed and high NOx emission represent significant challenges for combustor applications. The present study investigates the use of porous media burners (PMBs) to stabilize lean NH3/H2/air flames. By stabilizing the flame at the interface between two thermally conductive ceramic foams, internal heat recirculation preheats the reactants and increases the flame speed, which increases the combustion stability limits to very lean NH3/H2/air mixtures. Experimental studies are conducted to determine the stability limits of porous media combustion over a wide range of NH3/H2 volume ratios, equivalence ratios and mass fluxes. The NH3/H2 ratio is found to play a significant role with a typical dependency of the equivalence ratio at extinction in the form ϕext∝XH2−0.3. NOx emissions were shown to reach their minimum when the burner was operated near its extinction limit and to increase with increasing mass flux rates. Because unburnt NH3 emissions were also quite low (1400 ppm) at these very lean conditions, this operating regime offers similar or better emissions compared to the commonly employed fuel-rich operation which has been suggested in the literature.

A numerical study on the combustion limits of mesoscale methane jet flames with hydrogen addition

A meso-scale jet flame model was established for the flame ports of domestic gas stoves. The influences of hydrogen addition ratio (β = 0%–25%) on the combustion limits were explored. The results show that with the increase of hydrogen addition ratio, the blow-off limit increases obviously, while the extinction limit decreases slightly, namely, the combustible range expands significantly. Quantitative analysis was carried out in terms of chemical effect and thermal effect. It was found that hydrogen addition will reduce O2 fraction in the pre-mixture for a constant equivalence ratio. Under near-extinction limit condition, since the flame is located at the nozzle exit, the external O2 cannot be entrained into or diffuse into the upstream of the flame, which leads to the decrease of reaction rate. However, for the near-blow-off cases, the external O2 can be entrained and diffuse into the flame, which compensates the difference of O2 content in the pre-mixture. Therefore, the combustion reaction is enhanced by hydrogen addition because more H radicals can be produced. In addition, as the flame is located closer to the tube with the increase of hydrogen addition ratio, heat transfer between flame and tube wall is augmented and the preheating of fresh mixture is strengthened by the inner tube wall. This heat recirculation effect becomes especially notable in low velocity cases. In conclusion, the extension of extinction limit by hydrogen addition is attributed to the thermal effect, while the increase of blow-off limit is mainly due to the intensification of chemical effect.

Fully-resolved 3D premixed H2/air flames in a micro-combustor partially filled with porous media: Effects of detailed pore structures

Micro-combustors filled with porous media allow dual pathways for heat recirculation, one from the solid matrix and the other the combustor wall, thus representing a potential solution for micro-combustion applications. In the case of porous micro-combustors, the pore size is nearly comparable to the characteristic diameter/height of the combustor, and therefore pore-scale flame characteristics need to be fully resolved, which however cannot be realized in the conventional volume-averaged models (VAMs). A 3D pore-scale model (PSM) is developed in the present study to simulate the premixed H2/air combustion in a planar channel (H = 1 mm) partially filled with porous media. Conjugate heat transfer between the combustor wall and reacting gases is considered. Two types of porous media are geometrically modelled, they are, in-line arranged alumina spheres (1-mm-diameter) and SiC foam, with the resulting porosities to be 0.48 and 0.95, respectively. The comparison of flame spatial features indicates that the 2D VAM fails to delineate both the flame position and the flame thickness (volume) in such small combustors. For the two types of porous media considered, the results of the 3D PSMs clearly explain the difference in flame stabilization mechanism inside the porous zone. Heat recirculation (through the solid matrix and the combustor wall) and heat losses in the flame zone are quantified through numerical integrations. It is found that under the specific conditions considered, the alumina spheres exhibit a higher blow-off limit, a lower flame temperature, a lower fraction of heat recirculation and a lower fraction of heat losses than the SiC foam does. The present study confirms the necessity of considering detailed pore structures in modelling porous micro-combustors, and in future studies a wider range of flow velocity will be modelled for more generalized conclusions.

Compact Decentral Façade-Integrated Air-to-Air Heat Pumps for Serial Renovation of Multi-Apartment Buildings

To address the huge market of renovation of multi-apartment buildings, minimal-invasive decentral serial-renovation solutions are required. One major challenge in the design of decentral heat pumps is to find the optimal balance between, on one hand, compactness and pleasant design, and on the other hand, efficiency and minimal sound emissions. A comprehensive holistic design and optimization process for the development of decentral heat pumps, from the component level, to the system level, and up to the building level, is developed. A novel façade-integrated speed-controlled exhaust air to supply air heat pump combined with a mechanical ventilation system with heat recovery and recirculation air was developed and simulated in a reference flat. Compared to a traditional supply air heat pump without recirculation, it shows only slight performance improvement, but allows significantly better thermal comfort and control, independently from the hygienic air flow rate and from the heating and cooling loads. Detailed measurement and simulation results are presented for several functional models with heating power of around 1 kW up to 2.5 kW. The design was optimized by means of CFD simulations to allow for low pressure drop, homogeneous flow, and low sound emissions. Moreover, mock-ups of innovative façade-integrated heat pump outdoor units are presented.

Experimental proof-of-principle of heat recovery and recirculation in a reciprocating steam engine. Applicability of the technology to present electricity generating power plants and estimation of the yearly world energy saving and reduction of greenhouse gas emission

The motivation for the present study is energy production from thermonuclear fusion, as discussed in recent works [Panarella, Phys. Essays 33, 283 (2020); 34, 256 (2021); Peretti et al., Phys. Essays 34, 596 (2021)]. The direction of research for the attainment of the milestone of fusion energy breakeven was analyzed in depth in those works. The path of increasing the efficiency of the energy input deposition was found to be favorable relative to the alternative path of increasing the fusion energy output in ever bigger machines, as pursued for the past seven decades by all major research programs. The input for the fusion machines is electrical energy, which is generated from conventional engines that convert heat to work. In a simulation study, it was found that the efficiency of these engines could be improved through heat recovery and recirculation without violating the second law of thermodynamics. However, an experimental proof-of-principle was required to conclusively prove what the simulation indicated to be possible. The present study reports on such an experimental confirmation. It demonstrates experimentally a novel thermodynamic cycle where heat is re-used and re-circulated in a reciprocating steam engine. An advanced study of the second law of thermodynamics is provided that justifies this experimental result, as well as its historical interpretation. Re-use and recirculation of heat in engines used in power plants all over the world leads to global energy savings, as well as to significant reductions of global greenhouse gas emissions. These are estimated on a yearly time-scale with the most recent data available. Their significance regarding mitigation of climate change is highlighted.

Numerical study on methane/air combustion characteristics in a heat-recirculating micro combustor embedded with porous media

Micro-combustor is a portable power device that can provide energy efficiently, heat recirculating is considered to be an important factor affecting the combustion process. For enhancing the heat recirculating and improving the combustion stability, we proposed a heat-recirculating micro-combustor embedded with porous media, and the numerical simulation was carried out by CFD software. In this paper, the effect of porous media materials, thickness and inlet conditions (equivalence ratio, inlet velocity) on the temperature distribution and exhaust species in the micro combustor are investigated. The results showed that compared with the micro combustor without embedded porous media (MCNPM), micro-combustor embedded with porous media (MCEPM) can improve the temperature uniformity distribution in the radial direction and strengthen the preheating capacity. However, it is found that the embedding thickness of porous media should be reasonably arranged. Setting the thickness of porous media to 15 mm, the combustor can obtain excellent comprehensive capacity of steady combustion and heat recirculating. Compared the thermal performance of Al2O3, SiC, and ZrO2 porous media materials, indicating that SiC due to its strong thermal conductivity, its combustion stabilization and heat recirculating capacity are obviously better than that of Al2O3 and ZrO2. With the porous media embedded in the micro combustor, the combustion has a tempering limit of more than 10 m/s, and the flame is blown out of the porous media area over 100 m/s. The reasonable equivalence ratio of CH4/air combustion should be controlled within the range of 0.1–0.5, and “super-enthalpy combustion” can be realized.

Ultra-lean dynamics of a holder-stabilized hydrogen enriched flames in a preheated mesoscale combustor

To provide the theoretical basis to suppress the unstable flames under the coupling effect of flow and heat recirculation, the present work experimentally studies the ultra-lean dynamics of a holder stabilized 40%H2–60%CH4–air premixed flame in a preheated mesoscale combustor. The regime diagram of the flame behaviors at various operating conditions is obtained. It is observed that the blow-off limit first increases slightly and then decreases sharply (the anomalous blow-off limit) with the decreased Re value. Three types of the flame behaviors (i.e., the conventional stable flame, the stable residual flame, and the periodic oscillating residual flame) are found before the flame blow-off. In addition, with the decreased Reynolds number, the operating range for the stable residual flame broadens first and then narrows, but that of the periodic oscillating residual flame decreases monotonically, which are observed for the first time. The results show that, with the decreased Reynolds number, the flame root of the conventional stable flame anchors almost at the same location right behind the holder, while the flame tips obviously shift upstream. With the decreased equivalence ratio, the left and right flame tips in the downstream channel shift toward each other and finally merge into a single flame tip, which results in the formation of the stable residual flame. When the equivalence ratio decreases further, the periodic oscillating residual flame occurs. The flame tip periodically oscillates up and down over time. In the end, the blow-off dynamics of the stable residual flame and periodic oscillating residual flame are revealed.

Asymptotic study of premixed flames in inert porous media layers of finite width: Parametric analysis of heat recirculation phenomena

In this paper, we present an investigation on super-adiabatic premixed flames in an inert porous medium layer of finite length. The combustion process is modeled by a one-step Arrhenius kinetics in which density variations are taken into account. The asymptotic case of large ratio of thermal conductivities of solid to gas phases and the high activation energy limit are explored. The results obtained for these limiting cases are compared to those for finite values of these parameters. The use of the flame sheet model allows us to obtain steady-state solutions in an analytical form thus facilitating the parametric analysis.The investigation focuses on the phenomenon of multiplicity of steady-state solutions. It is shown that two or three (nontrivial) steady-state solutions are possible, depending on the flow rate intensity. The critical values of the parameters for the existence of multiple solutions are also determined. The stability of the obtained solutions is finally investigated by means of time-dependent simulations.

Enhancement of partial oxidation reformer by the free-section addition for hydrogen production

Hydrogen production by the partial oxidation combustion in porous media provides an efficient utilization for renewable biogas. In this paper, the free section with lengths of 0, 5, 10, 15 mm were designed in the middle of two-section porous media respectively. The effects of free section addition on the temperature distribution and methane conversion were investigated to determine the optimal burner parameters at the operating conditions of different velocities and equivalence ratios. Results indicate that the highest temperature appeared in the free length of 10 mm but the maximum of H2 energy conversion efficiency was in that of 15 mm. For the Al2O3 ceramic foam of 20 PPI, the better heat recirculation and preheating effect were shown between the gas and solid. However, the productions of H2 and CO at the 10 PPI burner reached the highest. With the increasing of equivalence ratio from 1.4 to 1.8, the energy conversion efficiency of syngas rose from 41% to 61% with the 15 mm free length. The appropriate free section addition contributes to improving the reforming efficiency, which provides a new way for the burner design.

Numerical Study on the Effect of Wall Thickness on the Combustion Characteristics of Non-premixed Hydrogen Micro-jet Flame

The effect of wall thickness on the combustion characteristics of nonpremixed hydrogen micro-jet flames was studied by two-dimensional numerical computation with a detailed reaction kinetics for the development of hydrogen micro-burner. The hydrogen jet diffusion flames achieved by micro-tubes with the same inner diameter and length but different wall thicknesses were investigated. The heat exchange between solid tube and gases were included in the numerical computation. The distributions of flame temperature, OH radicals, details of thermal interaction, and combustion efficiency were analyzed for comparison. It was found that the temperature distribution, flame shape, and heat recirculation are changed with the fuel flow velocity, and they are affected by wall thickness. The mechanism of wall thickness on the combustion characteristics of hydrogen jet diffusion flame was clarified. Finally, an interesting numerical experiment was conducted to give a further explanation of the effect of heat recirculation and to provide guidance of the thermal management of the micro-burner.

Towards Heat-Recirculation-Aware Virtual Machine Placement in Data Centers

As customers take virtual machines (VMs) as their demands, high-efficient placement of VMs is required to reduce the energy consumption in data centers. Existing Virtual Machine placement (VMP) strategies can minimize energy consumption of data centers by optimizing resource allocation in terms of multiple physical resources (e.g., memory, bandwidth, CPU, etc.). However, these strategies ignore the role of heat recirculation in the data center, which can cause a huge energy waste in cooling. To address this problem, we propose a heat-recirculation-aware VMP strategy for reducing the energy consumption of data centers. This novel VMP strategy takes into account heat recirculation coupled with multiple physical resource allocation to reduce the energy consumption of data centers. We design a simulated annealing based algorithm called SABA to lower the energy consumption of data centers where multiple VMs are deployed. SABA remarkably cuts down the energy consumption of physical resources through two salient features. First, it obtains an approximation of the optimum with much fewer iterations than simulated annealing algorithm (SA). Second, it reduces the number of activated servers required for VM tasks. We quantitatively evaluate the performance of SABA in terms of algorithm efficiency, the number of activated servers and the energy-saving. We compare the performance of SABA with state-of-art XINT-GA, PPVMP, TSTD and SA algorithms. Moreover, we evaluate the efficiency of SABA by leveraging a real-world 50 hours trace from practical IBM cloud data centers. Experimental results indicate that our heat-recirculation-aware VM placement strategy provides a powerful solution for improving the energy efficiency of data centers (SABA improves energy efficiency of cooling by up to 13.2% over TSTD, 13.8% over XINT-GA and 45% over PPVMP algorithm).