Hot Gas Temperature Research Articles

Transient Thermal Analysis of a Kiln Using Different Numerical Approaches by Means of CFD

A transient study for the prediction of the temperature of hot gases inside a kiln loaded with bricks, using Computational Fluid Dynamics (CFD), is presented in this work. The prediction of transient thermal performance is obtained through four different approaches: the Properties Fitted to Temperature Polynomials (PFTP) model, the density changes according to the Boussinesq Approximation (BA) model with a Constant Thermal Expansion Coefficient (CTEC), the BA model with a Variable Thermal Expansion Coefficient (VTEC) and the density changes according to the Ideal Gas Law (IGL) model. Furthermore, the effects of a layer of ceramic fiber as insulation are investigated. The numerical results of the transient temperature of the hot gases inside the kiln were validated with experimental data. The mean relative error of the transient temperature was 5.0%, 32.6%, 26.4% and 4.4% for the PFTP model, BA model with a CTEC, BA model with a VTEC and IGL model, respectively. It was concluded that IGL model is the best approach to predict the transient temperature of the kiln. Finally, an increase in the temperature of the hot gases of 12.7% was obtained by applying the IGL model and using a layer of 0.07 m of ceramic fiber as insulation. The results of this work could be helpful to improve the design and performance of kilns in further works.

Improving operation flexibility of the gas turbine through coolant intercooling and air injection using a sub-compressor

The use of renewable energy, particularly solar and wind power, is increasing, but it is necessary to alleviate the variation of such resources, a key drawback of using renewable energy. In this context, gas turbines capable of fast startup and rapidly changing power following a load have attracted considerable attention. Nevertheless, enhancing the operational flexibility of gas turbines is crucial to ensure their stable operation under various conditions. Several studies have attempted to enhance the flexible operating capabilities, by examining the operational stability assurance under a variety of conditions and life cycle increases by changing the operating conditions. This paper proposes a new gas turbine configuration and operating strategy in which a sub-compressor is used to increase the ramp rate and mitigate the thermal load of the turbine blades. The performance and dynamic behavior of the novel gas turbine was simulated using an in-house program based on the gas path analysis. Specifically, a sub-compressor with an inter-cooler was used to cool the coolant from 398 °C to 300 °C, which increased the gas turbine power by 1.3 MW and efficiency by 0.4%p compared to the conventional gas turbine. In addition, when applying a new operating strategy during the ramp-up phase, a part of coolant air was injected into the combustor to suppress the maximum turbine inlet temperature by 15 °C compared to the conventional gas turbine. As a result, the ramp rate could be doubled (from 18.75 to 37.5 %/min) by alleviating the fluctuations of hot gas temperature in the ramp-up phase.

Thermal recycling analysis in regenerative cooling channels based on liquid rocket engine cycles

For rocket cycles involving electric pumps, gas generators, expanders, and staged combustion, a thermal recycling method is proposed to provide a more realistic simulation of regenerative cooling systems in liquid rocket engines. This method simulates a real situation where the outlet of the cooling channel is thermally recycled through the turbine or preburner to the inlet of the combustion chamber. Supercritical fluid properties are determined using the extended Redlich–Kwong (RK)–Peng–Robinson (PR) real-fluid equation of state. The axisymmetric reacting flows of a thrust chamber with regenerative cooling channels are simulated using the flamelet-based lookup table. To account for the multi-injector effect in the two-dimensional axisymmetric simulation, a non-uniform velocity distribution is implemented, utilizing exponential distributions of each injector’s mass flow rate. For the hot gas temperature, coolant temperature, and cooling mass flowrate, the thermal recycling method is comparatively analyzed with the thermal decoupling method. The regeneration effect of the heated fuel is explored by evaluating the inflow energy, reaction energy, wall heat transfer, and the exit kinetic energy conversion. The thermal recycling method is developed for regeneratively cooled rocket engines with the expander, gas generator, staged combustion, and electric-pump cycles. Through this numerical procedure, the thermal recycling method is successfully applied to a liquid oxygen/methane engine and NASA’s Crew Exploration Vehicle nozzle with two types of cooling. Based on the results of the four types of rocket cycles, it is confirmed that the specific impulse increases by 1.5–2% due to the regenerative heat effect.

Green Solutions in the Fire Safety Strategy within a Medical Centre

This study investigates the behaviour of the hot gas temperature and associated smoke numerically in case of fire inside a medical centre. Fire Dynamics Simulator (FDS), provided by NIST, has been used to run the simulations for all proposed cases. Different solutions are suggested to reduce the severity of the hot gas temperature and smoke thickness, in case of fire inside the building. Among all investigated solutions, natural ventilation can do the best job. A mechanical fan is also an option, but not as good as a natural vent found at the top of the building. Natural ventilation can offer cheap and clean solutions to improve the safety level and environmental conditions inside the building, which offers an appropriate evacuation process for firefighters and civilians inside the building.

Hot Gas Outflow Properties of the Starburst Galaxy NGC 4945

We analyze 330 ks of Chandra X-ray imaging and spectra of the nearby, edge-on starburst and Seyfert type 2 galaxy NGC 4945 to measure the hot gas properties along the galactic outflows. We extract and model spectra from 15 regions extending from −0.55 to +0.85 kpc above and below the galactic disk to determine the best-fit parameters and metal abundances. We find that the hot gas temperatures and number densities peak in the central regions and decrease along the outflows. These profiles are inconsistent with a spherical, adiabatically expanding wind model, suggesting the need to include mass loading and/or a nonspherical outflow geometry. We estimate the mass outflow rate of the hot wind to be 1.6 M ⊙ yr−1. Emission from charge exchange is detected in the northern outflow, and we estimate it contributes 12% to the emitted, broadband (0.5–7 keV) X-ray flux.

Investigation of flashover occurrence criterion based on thermal equilibrium theory

Flashover is the rapid transition to a state of total surface involvement in a fire of combustible material within an enclosure fire, which is the most serious and fatal event in building fires. Although previous studies have provided quantitative and qualitative standards for flashover, the criteria of flashover were normally based on the independent measurement of experimental parameters and phenomena rather than the underlying physical reasons, resulting in a lack of mutual relationships between these criteria and the inability to reflect physical mechanisms. Moreover, the flashover criteria vary for different compartment fire fuels, which directly affects the calculation of compartment fire theory. This work was performed to provide a unified principle on the criteria of flashover based on the thermal equilibrium theory. The critical conditions for flashover are quantitatively developed by establishing a partial differential equation for the energy generation rate and energy loss rate with respect to the temperature inside the compartment. Moreover, based on the partial differential equation, inequalities were established for the relationship between the temperature of the hot gas and the heat flux of the floor at the onset of flashover, which was validated by the compartment experiment data. It was found that due to differences in emissivity and net heat transfer rate, at the onset of flashover, the hot gas temperature and the radiation heat flux at the floor were different for different fuels, and a high hot gas temperature does not necessarily indicate a high radiation heat flux at the floor.

Pressure Behaviour of Hot Gases and Smoke in Fires of Large Enclosures with Different Ventilation Systems

This study investigates numerically the pressure behavior of hot gases and smoke in fires of large enclosures. It mainly covers the effectiveness of using natural and mechanical ventilation in changing the pressure behaviour of hot gases and smoke inside different structures. Fire Dynamics Simulator (FDS), provided by NIST, has been used to run the simulations for all proposed cases. For the assumed scenarios, the values of hot gas pressure and temperature are extracted at different points, especially in the hot layer close to the ceiling, for further analysis. The effect of changing the location of the fire source has been investigated, too. Final results show noticeable changes in the behavior of pressure, and temperature, in the presence of different ventilation systems, both natural and mechanical. Thus, appropriate ventilation systems can improve the pressure behaviour inside the place, which offers a better environment and higher safety level, in case of fire, in such large enclosures, open plan offices,

Parameters effect and optimization on calciner for treating magnesium salt based on computational fluid dynamics

The magnesium salt Mg(NO3)2·2H2O from laterite nickel ore nitric acid leaching process can be reused by thermal decomposition using calciner. To improve the performance of the calciner, a new numerical model integrating the gas-particle hydrodynamics and the particle pyrolysis reaction was established by Euler-Lagrange approach. In particular, the particle size variation caused by pyrolysis reaction was considered to accurately describe the gas-particle hydrodynamics. The particle pyrolysis model was developed to realize the interphase mass transfer. The P-1 model was modified to realize interphase radiation heat transfer. Then, the influence of different parameters on the thermal decomposition processes was discussed by using the established model. Finally, the optimal conditions for Mg(NO3)2·2H2O thermal decomposition were determined via response surface optimization analysis. The results show that the thermal decomposition of Mg(NO3)2·2H2O can be improved by increasing the initial hot gas temperature, reaction zone height, and decreasing treatment capacity of Mg(NO3)2·2H2O. The degree of influence of each parameter on thermal decomposition follows the order of initial hot gas temperature > treatment capacity of Mg(NO3)2·2H2O > reaction zone height. The decomposition rate of 99.99 % is obtained at optimized conditions of initial hot gas temperature 1111 K, Mg(NO3)2·2H2O treatment capacity 251 kg/h and reaction zone height 9 m.

X-ray scaling relations of early-type galaxies in IllustrisTNG and a new way of identifying backsplash objects

ABSTRACT We investigate how feedback and environment shapes the X-ray scaling relations of early-type galaxies (ETGs), especially at the low-mass end. We select central-ETGs from the TNG100 box of IllustrisTNG that have stellar masses $\log _{10}(M_{\ast }/\mathrm{M_{\odot }})\in [10.7, 11.9]$. We derive mock X-ray luminosity (LX, 500) and spectroscopic-like temperature (Tsl, 500) of hot gas within R500 of the ETG haloes using the MOCK-X pipeline. The scaling between LX, 500 and the total mass within 5 effective radii ($M_{5R_{\rm e}}$) agrees well with observed ETGs from Chandra. IllustrisTNG reproduces the observed increase in scatter of LX, 500 towards lower masses, and we find that ETGs with $\log _{10} (M_{5R_{\rm e}}/\mathrm{M_{\odot }}) \leqslant 11.5$ with above-average LX, 500 experienced systematically lower cumulative kinetic AGN feedback energy historically (vice versa for below-average ETGs). This leads to larger gas mass fractions and younger stellar populations with stronger stellar feedback heating, concertedly resulting in the above-average LX, 500. The LX, 500–Tsl, 500 relation shows a similar slope to the observed ETGs but the simulation systematically underestimates the gas temperature. Three outliers that lie far below the LX–Tsl relation all interacted with larger galaxy clusters recently and demonstrate clear features of environmental heating. We propose that the distinct location of these backsplash ETGs in the LX–Tsl plane could provide a new way of identifying backsplash galaxies in future X-ray surveys.

Numerical simulation of steel-copper-steel composite cooling staves: Heat transfer characteristics and structural optimization

The heat transmission properties of the cooling stave were examined using a numerical simulation approach and compared with copper cooling stave and copper steel composite cooling stave under different furnace conditions and different structures. It is found that the existence of the steel layer greatly raises the maximum temperature of the cooling stave when no slag is present, and its hot surface temperature is 152 °C greater than that of the copper cooling stave. However, under typical operating conditions, the hot surface temperature of the steel layer is still below its limit working temperature of 470 °C. The temperature of the cooling stave rises with the temperature of the gas and the temperature of the inlet water and reduces with increasing water inlet velocity and slag thickness. To efficiently lower the temperature of the hot surface steel layer, the operation should ensure that the hot surface gas temperature is kept within 1300 °C, the cooling water speed is raised to 2.4 m/s, and the slag thickness should be set to 15 mm or higher. The optimal construction for a steel-copper-steel composite cooling stave is to keep the thickness of the steel layer at 11.9 mm and the thermal conductivity around 40 W/(m·K).

A Chandra Study of the NGC 7618/UGC 12491 Major Group Merger at Apogee: Multiple Cold Fronts, Boxy Wings, Filaments, and Arc-shaped Slingshot Tails

Analyses of major group mergers are key to understanding the evolution of large-scale structure in the Universe and the microphysical properties of the hot gas in these systems. We present imaging and spectral analyses of deep Chandra observations of hot gas structures formed in the major merger of the NGC 7618 and UGC 12491 galaxy groups and compare the observed hot gas morphology, temperatures, and abundances with recent simulations. The morphology of the observed multiple cold front edges and boxy wings are consistent with those expected to be formed by Kelvin–Helmholtz instabilities and gas sloshing in inviscid gas. The arc-shaped slingshot tail morphologies seen in each galaxy suggest that the dominant galaxies are near their orbital apogee after having experienced at least one core passage at a large impact parameter.

Conjugate Heat Transfer Evaluation of Turbine Blade Leading-Edge Swirl and Jet Impingement Cooling With Particulate Deposition

Abstract Internal cooling structures for gas turbine engines are becoming more complicated to push the hot gas temperature as high as possible, which, however, allows particulates drawn into the coolant air to be more readily to deposit within these passages and thus greatly affect their flow loss and thermal performance. In this study, internal swirl cooling and jet impingement cooling subjected to particulate deposition were evaluated and compared using a conjugate heat transfer method, with an emphasis on the thermal effects of the insulative deposits. To accomplish the goal, an unsteady conjugate mesh morphing simulation framework was developed and validated, which involved particle tracking in an unsteady fluid flow, particle–wall interaction modeling, conjugate mesh morphing of both fluid and solid domains, and a deposit identification method. The swirl and the jet impingement cooling configurations modeled the internal cooling passage for the leading-edge region of a turbine blade and were investigated in a dust-laden coolant environment at real engine conditions. Coupling effects between the dynamic deposition process and the unsteady flow inside the two cooling channels were examined and the insulative effects of the deposits were quantified by comparing the temperatures on the external and internal surfaces of the metal channel walls, as well as on the deposit layers. Results demonstrated the ability of the newly developed, unsteady conjugate simulation framework to identify the deposits from the original bare wall surface and to predict the insulation effects of the deposits in the dynamic deposition process. The dust almost covered the entire impingement channel, while deposits were only seen in the vicinity of the jets in the swirl channel. Despite this, a dramatical decrease of convection heat transfer was found in the swirl channel because the swirling flow was sensitive to the interruption of the deposits. In contrast, the deposits improved the heat transfer rate in the impingement channel. When the thermal effects of the deposit layer were taken into account, the wall temperatures of both two cooling geometries were substantially elevated, exceeding the allowable temperature of the metal material. Due to the denser deposit coverage, the impingement channel wall had a greater temperature increase than the swirl channel. In terms of flow loss, the presence of the deposits inhibited the swirl intensity by interrupting the swirling flow and thus reduced the friction loss, whereas the pressure loss was improved by the deposits in the impingement cooling.

Atomization of Borosilicate Glass Melts for the Fabrication of Hollow Glass Microspheres

Direct atomization of a free-flowing glass melt was carried out using a high-speed flame with the aim of producing tiny, self-expanding glass melt droplets to form hollow glass microspheres. Atomization experiments were carried out using a specially adapted free-fall atomizer in combination with a high-power gas burner to achieve sufficient temperatures to atomize the melt droplets and to directly expand them into hollow glass spheres. In addition, numerical simulations were carried out to investigate non-measurable parameters such as hot gas velocities and temperatures in the flame region by the finite volume-based software Star CCM+® (v. 2022.1.1), using the Reynolds-Averaged Navier–Stokes (RANS) turbulence and the segregated flow model. To calculate the combustion process, the laminar flamelet method was used. The experiments and simulations indicated that a maximum gas velocity of about 170 m/s was achieved at the point of atomization in the flame. The particle size distribution of the atomized glass droplets, either solid or hollow, ranged from 2 µm to 4 mm. Mean particle sizes in the range of 370 µm to 650 µm were highly dependent on process parameters such as gas velocity. They were in good agreement with theoretically calculated median diameters. The formation of hollow glass microspheres with the proposed concept could be demonstrated. However, only a small fraction of hollow glass spheres was found to be formed. These hollow spheres had diameters up to 50 µm and, as expected, a thin wall thickness.

Hydrogen sulfide removal from simulated synthesis gas using a hot gas cleaning system

This study investigated the removal, adsorption, and regeneration performances of simulated H2S produced from rice straw gasification in a hot gas cleaning system with combination of three types of adsorbents including zeolite, calcined dolomite and activated carbon. These experiments were conducted at 250 ℃ hot gas cleaning system and simulated synthesis gas containing hydrogen sulfide (H2S) concentration at 200 ppm. The experimental results indicated that the tested H2S removal efficiency and adsorbed capacity were observed to be adsorbed in decreasing order: activated carbon > calcined dolomite > zeolite. The adsorption capacities of zeolite, calcined dolomite, and activated carbon are 2.22, 3.16, 5.88 mg S/g adsorbents, respectively. In this research, the calcined dolomite shows poor adsorption-regeneration ability due to its deactivation by H2S. However, the tested zeolite and activated carbon could be fully regenerated at 350 °C with a stable sulfur adsorption capacity during four adsorption-regeneration cycles. These results gained from this study could significantly support researchers obtaining more information about the synthesis gas contaminants removal during hot gas temperature operation process.

Numerical Study on the Effect of Atmospheric Wind on the Fire Severity in Informal Settlements with Different Dwellings’ Wall Thermal Characteristics

AbstractThere is a persistent risk of large-scale fire conflagrations in informal settlements, which can threaten hundreds of people simultaneously. Although the literature implies that wind conditions have a significant impact on these fires, little is known about how wind conditions affect the dynamics and spread of flames in informal settlements. In order to comprehend the impact of wind conditions (speed and direction) on the time to flashover and fire severity in informal settlement dwellings with different wall thermal characteristics, a numerical study was conducted utilizing the Fire Dynamics Simulator (FDS), a Computational Fluid Dynamics (CFD) code. For six different wind speeds (1 m/s, 5 m/s, 10 m/s, 15 m/s, 20 m/s and 25 m/s) and two wind directions (side and back wind). Simulations were conducted with full-scale informal settlement dwellings burning wood cribs, analyzing the fuel mass loss rate, hot gas temperature, global equivalence ratio, radiative heat flux outside the door, and time to flashover. In addition, the influence of wall thermal properties was examined for thermally-thin steel-clad and asbestos cement-clad dwellings (thermally-thick). Regardless of wind direction, it was noticed that an increase in wind speed significantly shortened the time required to attain flashover. This was shown to be the result of the wind accelerating the burning rate of the wood cribs and, as a result, the faster temperature rise of the hot gas. Radiative heat fluxes observed outside the door increased with the wind speeds. The direction of the wind had a small effect on the investigated fire characteristics, with the side wind scenarios exhibiting somewhat longer timeframes to flashover. Thermally-thin walled informal settlement dwellings exhibited a greater fire severity, with higher fuel mass loss rates, hot gas layer temperatures, and higher external radiant heat fluxes, as well as shorter timeframes to flashover. These findings indicate that both wind speed and thermal wall characteristics have a substantial impact on the severity of fires in informal settlements and can enhance the risk of fire spread.

Analytical investigation of free piston Stirling engines using practical stability method

The existence of a stable limit cycle in the dynamical system (sufficient condition) of free piston Stirling engines (FPSEs) is the main challenge for designers to achieve. Therefore, this paper provides a novel analytical and parametrical scheme based on the practical stability and dynamic error to evaluate this issue. In this regard, the dynamic error equations of the engine are first extracted based on the target design criteria. Afterward, the practical stability theorem is implemented on the dynamic error of the engine. The main finding of this approach has led to the provision of nine parametric conditions to satisfy the sufficient condition (i.e. the most important condition for a stable oscillation) of the FPSE. Indeed, these conditions help the designers to specify the allowable ranges for the engine parameters in which the sufficient condition is met for the first time. In other words, a novel parametrical design criterion is provided for FPSE. In addition to this important achievement, this novel technique has been able to present a design criterion (value of ultimate bound) to evaluate the design accuracy for the first time. According to this design criterion, minimizing the ultimate bound value results in a more accurate engine design. In this regard, increasing the power piston mass, hot gas temperature, and mass of the gas inside the engine helps the designers get closer to their design targets. Besides, increasing the value of stiffness and mass of the displacer piston creates an optimum condition in which the value of the ultimate bound of the engine is reached its minimum amount. Finally, the presented method is then evaluated through the specifications of two experimental case studies, including SUTech-SR-1 and B10-B. The outcomes showed that this technique could well predict the sufficient condition of the FPSEs.

Correction formula for film-cooling effectiveness considering the influence of thermal radiation in high-temperature environments

Film-cooling effectiveness (η) evaluation test of aero-engine turbine blades is usually conducted in a low or medium temperature environment. Through the scaling criteria, it can be approximately equivalent to the η in a real high-temperature environment. However, this method does not consider the influence of thermal radiation and overestimates the cooling effect of the gas film in a high-temperature environment, which in turn affects the design accuracy of turbine blades. In this study, the thermal radiation correction of the film-cooling effectiveness scaling criteria was done by developing a physics-based empirical correlation. Firstly, the radiation correction factor (krad) was defined as the average value of the flow direction of η¯high/η¯low, which is related to gas-to-cold-air temperature ratio (TR), emissivity (εw), and operating temperature ratio (the ratio of the hot gas temperature of the η¯ predicted to that of the known η¯, Tbi). Secondly, based on computational fluid dynamics (CFD) results, the influence of the dimensionless parameters (εw, Tbi, TR) on krad was revealed. Subsequently, the physical relationship and correlation form between the three parameters and krad was preliminarily determined. Moreover, a test dataset (εw, Tbi, TR, krad) of a 3 × 4 × 3 measurement matrices was built. The relationship between (εw, Tbi, TR) and krad was incorporated into the empirical framework of krad, and the empirical coefficient was determined by nonlinear fitting. Finally, the applicable dimensionless parameter range for the current correlation were determined by recording datasets with the coefficients of determination (R2) > 0.8. When the scaling criteria from low to high temperature were satisfied and the influence of the radiation participating medium was ignored, εw was in a range of 0.3–0.8, Tbi was in a range of 1–3, and TR was in a range of 1.5–2.5; any reasonable Reynolds number and blowing ratio were applicable to the thermal radiation correction correlation.

Experimental research on enhanced heat transfer of double-pipe exchanger with audible acoustic field

Acoustic reinforced heat transfer is an active reinforced heat transfer method that has the advantages of having no contacts and being high energy and adjustable. To explore the convective heat transfer characteristics under the audible acoustic field, a convective heat exchange experimental platform was built; the platform was a double-pipe heat exchanger. Comparative experimental research on heat transfer of hot flue gas both with acoustic field and without acoustic field was carried out. The results verified that the heat exchanger with an acoustic field had better heat transfer effect. The effects that parameters (such as sound pressure level, acoustic frequency, and the inlet temperature of the hot flue gas) had on the enhanced heat exchange effect were systematically studied. The influence law of the acoustic field on the convective heat transfer capacity of heat exchanger was obtained. The results show that, with an acoustic field, the temperature of the hot flue gas at the outlet of the heat exchanger decreases significantly, whereas the heat transfer power, convective heat transfer coefficient, and the Nusselt number increased significantly. Within the selected range of experimental parameters, 0.6 kHz is the optimal acoustic frequency for enhanced heat transfer. With an increase in the sound pressure level, the reinforcement efficiency increased. At the same time, increasing the inlet temperature of the hot flue gas helped to enhance the capacity of the sound wave to strengthen the heat transfer. When the temperature of the inlet hot flue gas was 59 °C, the sound pressure level was 145 dB, the acoustic frequency was 0.6 kHz, and the maximum reinforcement efficiency was 76.22%.

Characteristic simulation and working parameter optimization of the aggregate dryer

The aggregate dryer is generally used in an asphalt mixing plant for road construction equipment and serves to heat the aggregates. In this work, the coupled method of Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) is used to study the gas-solid coupling of aggregate dryers to improve the heat exchange efficiency between flue gas and aggregate. The heating process of aggregates in an aggregate dryer is visualized by analyzing the particle discrete field and the fluid temperature field during drying. The motion process and distribution characteristics of aggregate particles are established for different outputs and rotational speeds, and the hot flue gas distribution and temperature characteristics of the aggregate dryer are also obtained. The results show that the rotational speed affects the distribution of the aggregate veiling zone and pressure, which in turn affects the stability of the flame. Reducing the rotational speed results in a more uniform distribution of the aggregate veiling zone and thus an increase in heat utilization. In addition, when the output of the aggregate dryer is 80 t/h and the rotation speed is 6.2 rpm, the aggregate discharge temperature is the highest and the thermal efficiency of the aggregate dryer is the best.

Simulation view of galaxy clusters with low X-ray surface brightness

Context. X-ray selected samples are known to miss galaxy clusters that are gas poor and have a low surface brightness. This is different for the optically selected samples such as the X-ray Unbiased Selected Sample (XUCS). Aims. We characterise the origin of galaxy clusters that are gas poor and have a low surface-brightness by studying covariances between various cluster properties at fixed mass using hydrodynamic cosmological simulations. Methods. We extracted ≈1800 galaxy clusters from a high-resolution Magneticum hydrodynamic cosmological simulation and computed covariances at fixed mass of the following properties: core-excised X-ray luminosity, gas fraction, hot gas temperature, formation redshift, matter density profile concentration, galaxy richness, fossilness parameter, and stellar mass of the bright central galaxy. We also compared the correlation between concentration and gas fractions in non-radiative simulations, and we followed the trajectories of particles inside galaxy clusters to assess the role of AGN depletion on the gas fraction. Results. In simulations and in observational data, differences in surface brightness are related to differences in gas fraction. Simulations show that the gas fraction strongly correlates with assembly time, in the sense that older clusters are gas poor. Clusters that formed earlier have lower gas fractions because the feedback of the active galactic nucleus ejected a significant amount of gas from the halo. When the X-ray luminosity is corrected for the gas fraction, it shows little or no covariance with other quantities. Conclusions. Older galaxy clusters tend to be gas poor and possess a low X-ray surface brightness because the feedback mechanism removes a significant fraction of gas from these objects. Moreover, we found that most of the LX covariance with the other quantities is explained by differences in the gas fraction.