Type Heat Exchanger Research Articles

Investigating the impact of different SCWSVGs arrangements on thermo-hydrodynamic flow within pillow-plate type heat exchanger

This paper introduces an innovative approach aimed at enhancing heat transfer while simultaneously reducing pressure loss in the Pillow-plate type heat exchanger (PPHE) through the utilization of circular welding spots (CWS) with a dual role as vortex generators (VG) and convergent/divergent features. The VGs, strategically positioned in a configuration termed “SCWSVGs” (Staggered circular welding spot vortex generators), are fixed in place to augment heat transfer. This configuration results in an amplified heat transfer coefficient, increased turbulence, and intensified vortices and secondary flow behind the SCWSVGs. The effectiveness of this enhancement strategy is validated through comparisons with experimental data, correlations, and various channel arrangements. Furthermore, the study investigates the impact of SCWSVGs diameters and streamwise pitches (XL) by exploring the variation in the hydraulic diameter to streamwise length ratio (dh/XL) within the range of 0.036 to 0.070. Key parameters in the dataset include Nu, f, (Cx/Cx0)0.33, normalized heat transfer factor (St/St0), RAF, and TEF. Notably, at 1,000 ≤ Re ≤ 16,000, the highest and lowest heat transfer rates are observed for cases 1–5 and cases 15–20, respectively. The (Nu/Nu0)/(f/f0)0.33 reaches up to 67 %, while f decreases by 9 % (Re = 1,000) to 10 % (Re = 16,000) based on two times the streamwise length (XL). Varied XL values result in significant differences in Nu, highlighting the superior performance of PPHE compared to a smooth enclosure channel.

Optimizing Plate Heat Exchanger Design for Steam Condensate Recovery Systems

Condensate water is typically generated as a by-product during the use of steam as a medium for heat transfer. In order to optimize the transfer area, the chemical industry often employs heat exchangers, such as the plate heat exchanger (PHE), which provides flexibility in adjusting the space area for heat transfer. This case study examines the optimization of plate geometry dimensions to enhance the heat transfer process of PHE equipment design. The optimization process involves mathematical equations and a literature review of the design of this type of heat exchanger. Improving the dimensional aspect of the plate geometry results in an increase in the overall heat transfer coefficient (U) and a reduction in the number of design requirements used. The study's estimation results suggest that the PHE design has a limitation on plate dimensions, which should be less than 0.3×0.6 m. Additionally, it is important to consider the pressure drop value, which should not exceed 29.07 kPa. A review of the chemical industry field provided estimated options for plate size and quantity, both of which support the optimization of heat transfer rate and design constraint thresholds. The implementation of the design has been found to enhance the performance of the planning process for recovering steam condensate water.

Experimental comparison of finned tube heat exchangers for heat rejection under natural convection conditions

This article presents experimental analysis of heat removal capabilities of a wavy finned tube heat exchanger operating under natural convection conditions Two types of heat exchangers are considered in this work: standard construction exchangers, most often found in waste heat rejection devices and exchangers based on a novel geometry, which supports intensification of heat transfer processes under natural convection. The aim of the research was to experimentally verify the novel conception and to determine the thermal efficiency of heat exchangers under the influence of selected design parameters. The factors analyzed in the research include: temperature difference between the heat transfer surface and the surrounding air, different angles of the units transferring heat and the height of the exchanger casing. Four variants of the casing height were considered in this work. The conducted research also allowed to determine the influence of the fin spacing on the performance of the heat exchangers under natural convection conditions. The test results obtained show that the novel geometry allows for significance improvement of heat transfer efficiency. The proposed solution allowed to increase the density of the heat flux from 12% in (fin spacing of 2.8 mm) up to 24% (fin spacing of 5.6 mm). It was also observed that the height of the casing has significant impact on the heat transfer efficiency. For each of the analyzed cases, the heat exchanger without a casing was characterized by the lowest thermal efficiency, while the highest heat flux was removed from the exchanger with the highest casing (up to four times higher heat transfer rate). It was determined that the largest amount of heat was removed from exchangers located horizontally, while the exchanger with an angle of 60°, was characterized by the lowest rate of heat removal. Another factor which was determined to have significant impact on the performance of the heat exchangers is the spacing between the fins. The research results obtained shows that increasing the spacing of the finned surface from 2.8 mm to 5.6 mm resulted in an average twofold increase in thermal efficiency in the case of four-row exchangers and an almost threefold increase in the case of two-row exchangers. The obtained results can be used as basic guide for the development of finned tube heat exchangers operating under natural convection conditions.

Parametric design optimization and thermodynamic analysis of plate fin heat exchanger for helium liquefaction system

The plate-fin heat exchanger is one of the most important components of helium liquefaction system as it recovers cold energy from helium gas in the cold box. Because of larger heat transfer surface area and compact geometrical configuration, plate-fin heat exchangers are widely used. The design of heat exchanger using helium as a cryogenic fluid needs high thermodynamic performance with minimum pressure drop which particularly requires the optimization of dimensional array and its geometrical configurations. In this regard, a parametric investigation was performed to compute the effective geometrical parameters and its effect on thermodynamic (effectiveness and log-mean temperature difference) performance has been discussed in detail. The present study aims to design optimization and thermodynamic performance evaluation of plate-fin type heat exchangers used in helium liquefaction systems. The developed helium liquefaction model has been simulated for the most critical liquefier with and without liquid nitrogen precooling, mixed mode, and refrigeration mode. Each component of the liquefaction model has been technically and thermodynamically analyzed and, in particular, four plate-fin heat exchangers have been considered and designed accurately using Aspen EDR®. Initially, the sensitivity analysis was carried out to characterize the most significant geometrical parameters (fin thickness, height, number and arrangement of layers, frequency, and fin type) and their effect on thermodynamic performance and pressure drop. After that, the artificial intelligence model was used to determine the optimal range of such geometrical variables in which the heat exchanger has maximum effectiveness and log mean temperature difference. Finally, the operating conditions obtained from cycle modeling and optimized geometrical datasets have been applied in commercial simulation software Aspen EDR® for plate-fin heat exchangers design. Finally, a comparative thermodynamic analysis has been carried out to determine the performance of all the designed heat exchangers operating at four different modes (boundary conditions).

Design and optimization analysis of a new double-layer tube type heat exchanger for lead-bismuth reactors

Double-layer heat tubes have been designed to effectively reduce the occurrence of heat pipe rupture accidents. However, inter-tube thermal contact resistance can decrease heat transfer efficiency, thus hampering the heat dissipation in the primary loop system of lead-bismuth reactors. Therefore, optimizing the design of double-layer heat tubes is necessary. This work focuses on the double-layer heat exchanger of a lead-bismuth reactor and utilizes gallium-based graphene nanofluids as a thermal interface material to fill the gap between the heat tubes. Furthermore, the impact of the length, wall thickness, outer diameter, and spacing of heat tubes on the heat transfer performance of the double-layer heat exchanger with and without the nanofluids has been analyzed. The study aims to optimize the JF factor and cost-effectiveness ratio (CER). Genetic algorithms are employed to optimize and evaluate the heat transfer performance of the main heat exchanger based on the four aforementioned parameters. Consequently, a new design scheme is obtained for the double-layer heat exchanger, which increases the optimized overall heat transfer coefficient of the main heat exchanger by 5.79%, pressure drop in the primary loop by 2.32%, JF factor by 5%, and CER by 24.62%. These results demonstrate that the gallium-based graphene nanofluids can effectively enhance the heat transfer performance of the double-layer heat exchanger while reducing the likelihood of steam generator tube rupture accidents.

Pengaruh Kenaikan Laju Alir Fluida Panas dan Arah Aliran terhadap Kinerja Plate and Frame Heat Exchanger

Plate and Frame Heat Exchanger (PFHE) is a plate and frame type heat exchanger that is efficient and effective in improving energy efficiency. Research using PFHE aims to determine the effect of increasing the flow rate of hot fluid on heat exchanger performance. This research was conducted at a flow rate variation of 0.7 L/min, 0.9 L/min, 1.1 L/min, 1.3 L/min, 1.5 L/min, 1.7 L/min, and 1.8 L/min with 180 seconds of testing each variation and recording data every 2 seconds. The cold fluid used is tap water and the hot fluid used is distilled water. The results showed that with the increase in hot fluid flow rate, the performance of PFHE also increased because the effectiveness of NTU (Ɛ-NTU) increased. The Ɛ-NTU value of unidirectional flow is 25.37% - 44.87% and in the opposite direction 37% - 68.39%. The largest Ɛ-NTU value in the countercurrent flow indicates that the countercurrent flow is more effective than the unidirectional flow. In addition, increasing the flow rate increases the Reynold's number (Nre) and Nusselt's number (Nu) which indicates the greatest convection heat transfer occurs at the highest flow rate.

Numerical study of functional parameters of compact heat exchanger with the corrugated wall in steady state

Compact heat exchangers have several benefits over other types of heat exchangers, including compactness, a high surface area to volume ratio, and a low-temperature differential. They are frequently employed in the heat integrating process. Herein, a compact heat exchanger is simulated in a steady state using the AnsysFluent 17.2 software pack. The flow regime is laminar and water enters the exchanger at a temperature of 298.15 K, assuming a constant wall temperature condition. This study examines the effects of wall wave formation on fluid flow and heat transfer characteristics. The two parameters, wave altitude, and wave angle, are verified and the resultant corrugation is fitted as a sinusoidal function. As the wave height factor increases, the results show that the temperature and velocity values in the solution field climb. More than 4.5% more has been added to the highest temperature value. It is also shown that the velocity values rise with the wave angle coefficient. The greatest rise is 50%. The Performance Ratio is also examined in this study. The findings show that the angle coefficient of 0.5π and π and the altitude coefficient of 0.2 provide values for the Performance Ratio that are higher than the unit.

Analysis of Fouling Factors on Heat Transfer in Shell and Tube Heat Exchangers Type 1-2 Pass

Analysis of Fouling Factors on Heat Transfer in Shell and Tube Heat Exchangers Type 1-2 Pass

An experimental method of determination and validation of latent thermal energy storage dynamic characteristic by effective enthalpy

The paper draws attention to the problems of evaluating stored heat capacity in a latent heat thermal energy storage (LHTES) that operates with unsteady inlet temperature conditions. These problems are studied for custom-built LHTES designed to be charged/discharged at varying heat transfer rates in a chilled water system. The LHTES incorporates a typical fin and tube heat exchanger and commercially available ATP 20 phase change material (PCM). For the designed storage construction, the ratio of heat transfer area to PCM volume is 354 m2/m3, and the ratio of PCM volume to the total volume (compactness factor) is 0.87. The LHTES thermal characteristics were investigated on a test stand using inlet temperature inputs changing linearly at 1, 2 or 3 K/h. During each test, a complete phase transition in the PCM was ensured. Based on LHTES inlet and outlet temperature profiles, enthalpy distributions versus outlet temperature were evaluated. The experiments proved that these distributions are not a material property as they depend on the geometric characteristics of the LHTES and charging/discharging rates. However, they can describe the thermal behaviour of the PCM filling the LHTES and include the influence of a non-uniform temperature distribution inside the LHTES on the PCM hysteresis and temperature range of the complete melting/solidification. These effects can be modelled by the proposed function of effective enthalpy. The effective enthalpy allows for relatively simple monitoring of phase transition progression in the LHTES based solely on the parameters measurable outside the storage.

Effect of SCWSP-VGs and thermal performance investigation of pillow-plate type heat exchanger (PPHX)

Vortex Generators (VGs) are an innovative heat transfer enhancement in traditional heat exchangers. With this technique, the study investigates the combined impact of upper-lower Staggered Circular Welding Spot Patterns (SCWSP) and VGs, termed SCWSP-VGs, in a periodic enhanced Pillow-plate type heat exchanger (PPHX) using ANSYS Fluent 18.1. Numerical results reveal SCWSP-VGs induce a periodic velocity profile and opposite elliptical recirculation vortex (OERV) turbulence, significantly improving thermo-hydrodynamics compared to smooth enclosures. In addition, the performance of the PPHX is described or presented in the graphical form in terms of Colburn j-factor, Nu/Nu0, f/f01/3, Reynolds analogy factor, normalized drag coefficient ratio (Cx/Cx0)0.33, and (Nu/Nu0)/(f/f0)1/3. The maximum Nu and performance evaluation criterion (PEC) are improved by 41.34% and 1.68 in the dh/XL = 0.076 and 0.043 at Re = 1000, respectively.

Review[sbnd]Recent developments on applying acoustic waves for efficiency improvements of different thermofluids systems

This review classified recent works using proper acoustic waves to develop more efficient thermofluids systems for different industrial equipment such as swirl combustor, jet in crossflow, gas flare, different types of heat exchangers, cooling mechanism, aircraft wings, and turbine blades. For the swirl combustor, the range of proper excitation Strouhal number and jet pulsation intensity depends on the type of fuel, geometry of burner, swirl number, and central and annular Reynolds numbers. The combined effects of early jet break up, and centrifugal force induced by proper acoustic waves can lead to higher jet spread rate that can induce higher levels of mixing of the fuel and air in swirl combustor, improving combustion efficiency. For the transverse jet, the range of proper excitation Strouhal number and jet pulsation intensity depends on the type of fuel, geometry, and angle and direction of the burner, and jet and cross flow Reynolds numbers. Proper acoustic excitation can force the transverse jet flow to synchronized shear layer vortices. This phenomenon can cause the new vortices to appear in the upwind shear layer region of the deflected jet; then the spread, penetration, entrainment, and dispersion of the jet significantly increases, improving combustion efficiency. For the aircraft wing, vertical acoustic excitation significantly improves the lift force when the sound frequency is equal to the period of flow fluctuation in the separated boundary layer. For the heat exchanger, the ultrasonic transducer can be installed on the walls of a heat exchanger with direct contact to the fluid, improving the heat transfer coefficient. The physics of fluids behind the efficiency improvements of all mentioned cases were also clarified. Moreover, the acoustically excited impinging jets with higher cooling efficiency applied in different areas are discussed. Proper acoustic forcing can decrease the rate of unstable frequencies of disturbances related to impinging jets over a short distance. This phenomenon can significantly enhance heat transfer and improve cooling efficiency. This review paper provides insight into how acoustic waves could improve the efficiency of thermofluids systems in a variety of applications. Acoustic excitation could have the potential capability to improve the efficiency of new applications such as wind, gas or steam turbines, some types of industrial chemical mixers with parallel plane jets, aircraft wings or those of unmanned aerial vehicles, more efficient combustion chambers, and less polluting gas flares. However, the feasibility of this concept requires further investigation by the community in the future.

Experimental investigation on the use of PCM heat exchangers in Geo-energy piles and walls

The current paper aims to experimentally investigate the thermal performance of geo-energy piles and walls fabricated with Phase Change heat exchangers. Four prototype concrete geo-energy structures (i.e., piles and walls) were tested using two distinct types of heat exchangers, including standard heat exchangers and PCM heat exchangers. The PCM heat exchangers utilized in the current study were filled up with two different types of Phase Change Materials (PCM) with melting points of 26 °C and 42 °C for geo-energy piles and walls, respectively. The thermal efficiency of the geo-energy piles/walls was experimentally assessed over 100 h of continuous operation under cycles of cooling and heating. The findings illustrated that using PCM heat exchangers led to enhancing the heat transfer efficiency of geo-energy piles by 75 % and 43 % in heating and cooling operations, respectively, compared to those achieved using a standard heat exchanger. Furthermore, the heat transfer performance of geo-energy walls with a PCM heat exchanger was enhanced by 43 % and 32 % in heating and cooling tests, respectively, compared to those achieved using a standard heat exchanger. Moreover, the findings indicated that the inclusion of PCM heat exchangers in geo-energy structures contributed to reducing: the impact on soil temperature and thermal interference radius as well as the potential structural damage due to thermal stress.

Numerical investigation of louver edges effect on the performances of louvered fin compact heat exchanger

The rectangular channel louvered-fin compact heat exchanger (LFCHE) is the most efficient type that has been served in a radiator. However, with this type of heat exchanger, the pressure drop rises by three to four times as the heat transfer increases, which results in decreasing performance. This research is aimed to numerically investigate the louver edges (vertical, inclined, and horizontal) effect on LFCHE performance at different louver angle (θ) with air inlet velocities ranging from 1 to 30 m/s. A total of twelve models are made and simulated. The result revealed that the horizontal edge decreases the pressure drop up to 24.2% with a 1.01% increase in outlet air temperature over the base model (inclined edge). Thus, louver edge has design which results in higher and lower effect on pressure drop and temperature change, respectively. The research investigated the effects of louver edges using different performance evaluation criteria. At 10 m/s (Relp = 972.33) the horizontal edge increases the volume goodness (jf) factor up to 21.49% over the base model. Similarly, the horizontal edged fins resulted in maximum increment of jf-factor by 22% and 25% as compared with inclined edge for louver angles θ of 24 ° (at low Relp) and 20 ° (at high Relp), respectively. Generally, the horizontally edged LFCHE is proven to have higher performance in cooling the coolant with a minimum air side pressure drop.

Study on the thermal performance of the cross-flow plate heat exchangers

The exhaust gas emitted by a stenter machine will lead to energy waste and environmental heat pollution. Therefore, designing an efficient waste heat recovery apparatus for exhaust gas not only protects the environment but also saves production costs. In the article, the heat transfer performance and pressure drop of three types of heat exchangers: flat-plate heat exchanger (PHE), wavy-PHE, and zigzag-PHE were numerically studied in the context of waste heat recovery from the exhaust gas of stenter machines. The heat transfer rate (Q), Nusselt number (Nu), Colburn factor (j), pressure drop, Fanning friction factor (f), and comprehensive performance factor (JF) were investigated while Reynolds numbers ranged from 1000 to 9000. An experimental measurement was devised to validate the accuracy of the numerical simulation. The results of the research indicate that the wavy-PHE exhibits improved heat transfer efficiency and reduced pressure drop, primarily attributed to the substantial generation of secondary flow. Three new correlations for Nusselt number in different PHEs were developed based on commonly existing empirical formulas. The prediction accuracy for the Nusselt number based on the new correlations has been significantly improved in the present study.

Study the effect of axially perforated baffle plate with multiple opposite-oriented trapezoidal flow deflectors in an air–water tubular heat exchanger

Purpose The research focused on analysing a unique type of heat exchanger that uses swirling air flow over heated tubes. This heat exchanger includes a round baffle plate with holes and opposite-oriented trapezoidal air deflectors attached at different angles. The deflectors are spaced at various distances, and the tubes are arranged in a circular pattern while maintaining a constant heat flux. Design/methodology/approach This setup is housed inside a circular duct with airflow in the longitudinal direction. The study examined the impact of different inclination angles and pitch ratios on the performance of the heat exchanger within a specific range of Reynolds numbers. Findings The findings revealed that the angle of inclination significantly affected the flow velocity, with higher angles resulting in increased velocity. The heat transfer performance was best at lower inclination angles and pitch ratios. Flow resistance decreased with increasing angle of inclination and pitch ratio. Originality/value The average thermal enhancement factor decreased with higher inclination angles, with the maximum value observed as 0.94 at a pitch ratio of 1 at an angle of 30°.

Efficiency Enhancement in Ocean Thermal Energy Conversion: A Comparative Study of Heat Exchanger Designs for Bi2Te3-Based Thermoelectric Generators.

This research focuses on enhancing the efficiency of Bi2Te3-based thermoelectric generators (TEGs) in ocean thermal energy conversion (OTEC) systems through innovative heat exchanger designs. Our comparative study uses computer simulations to evaluate three types of heat exchangers: cavity, plate-fins, and longitudinal vortex generators (LVGs). We analyze their impact on thermoelectric conversion performance, considering the thermal energy transfer from warm surface seawater to TEGs. The results demonstrate that heat exchangers with plate-fins and LVGs significantly outperform the cavity heat exchanger regarding thermal energy transfer efficiency. Specifically, plate-fins increase TEG output power by approximately 22.92% and enhance thermoelectric conversion efficiency by 38.20%. Similarly, LVGs lead to a 13.02% increase in output power and a 16.83% improvement in conversion efficiency. These advancements are contingent upon specific conditions such as seawater flow rates, fin heights, LVG tilt angles, and locations. The study underscores the importance of optimizing heat exchanger designs in OTEC systems, balancing enhanced heat transfer against the required pump power. Our findings contribute to a broader understanding of materials science in sustainable energy technologies.

Design and performance analysis of a discontinuous spiral baffle heat exchanger

In this paper, a new type of heat exchanger with discontinuous spiral baffles is designed, and its performance on heat transfer is analyzed by numerical simulation. The research results show that the airflow in shell-side flows by a similar helical way, “Flow dead zone” is not found. The high enthalpy airflow’s temperature decreases about 38% of the inlet air temperature by cooling down with a convective heat transfer coefficient of 344.4 W/(m2·K), while the decrease of the airflow pressure is 30%.

Investigation of Heat Exchanger Performance in The Heating Tank Section of Loop FASSIP 03 NT

The Passive System Simulation Facility (FASSIP) loop is an experimental test facility for a passive cooling system to recover the residual heat from decay produced by the reactor core during accident conditions. The Heating Tank Section (HTS) is one of the components of the FASSIP 03 NT facility. This component is equipped with a heat exchanger, 3 types of heat exchangers can be applied to HTS. Namely heat exchangers of the Straight Pipe Heat Exchanger (SPHE) type, the Straight Pipe Fins Heat Exchanger (SPFHE) type, and the Helical Pipe Heat Exchanger (HPHE) type. A modification was made to increase the efficiency of HTS, namely replacing the electric heater on the HTS from a ceramic band heater type to an immersion heater type. With this modification, it is necessary to know the performance of the heat exchanger on HTS and its speed in reaching operational temperature. The HPHE-type heat exchanger is more efficient than the SPHE-type and SPFHE-type heat exchangers. The HPHE-type heat exchanger has a much larger length of 5.5 m, so the thermal resistance (Rth) is very small, namely 0.003926 ℃/W. To reach the working fluid temperature in the range of 50 – 90 ℃, the HPHE-type heat exchanger requires 35 – 86 minutes.

Multi-Criteria Study on Ground Source Heat Pump with Different Types of Heat Exchangers

Heat pumps are currently one of the most frequently applied heat sources in residential buildings. Ground source heat pumps are more reliable than air source heat pumps in terms of energy efficiency, especially in colder climates. However, they are more expensive and involve increased material inputs; therefore, multi-criteria analyses taking into account environmental and economic aspects seem necessary for the green design of these systems. The aim of this work was to analyze the environmental and economic impacts of the ground source heat pump providing heating for a family house located in eastern Poland, cooperating with three types of ground heat exchangers (each in two sizing options): helix, vertical and horizontal. The multi-criteria analysis was based on the life cycle assessment methodology using IMPACT 2002+ and life cycle costs methods. The lowest environmental impact was reported for the variants with vertical ground heat exchangers, mainly due to their high efficiency in the operation stage. On the other hand, the lowest economic impact was observed for the horizontal heat exchangers, which are not demanding in terms of material and construction costs. Final recommendations based on multi-criteria analysis propose the vertical probes as a sustainable solution, with a weighted sum indicator in the range 0.085–0.297 on 0–1 scale.

Using different geometries on the amount of heat transfer in a shell and tube heat exchanger using the finite volume method

This research investigates the heat transfer and pressure drop characteristics of a conical spiral tube heat exchanger. The study specifically focuses on the application of Ag-HEG (Silver–Hydrogen exfoliated graphene) nanofluid and various turbulator designs. The range of Dean Numbers, specifically 2200 < Dean <4200, was studied through experiments under turbulent flow conditions. Furthermore, the finite volume method-based ANSYS Fluent commercial code was utilized for numerical simulations. Additionally, numerical simulations were performed using the ANSYS Fluent software, which utilizes the finite volume method. This research aims to perform a numerical analysis on the efficiency of a conical shell and tube heat exchanger. When compared to other models, spiral heat exchangers provide a larger contact area between the fluid and the exchanger within a specified occupied area. This advantage is one of the prominent features of this particular type of heat exchanger. The simulations were conducted in two stages. During the first stage, the thermal performance coefficients of three turbulators were evaluated. In the second stage, the four-blade turbulator with ten revolutions, which exhibited superior thermal performance, was further analyzed based on the number of circles around the center of the conical spiral coil. The numerical results showed that the four-blade turbulator with ten revolutions displayed superior thermal performance compared to the other modes, specifically the two-blade and three-blade turbulators. In the second stage, it was found that the Nusselt number achieved from 30 revolutions was higher by 4.2 %, 10 %, and 18.3 % compared to the Nusselt numbers obtained from the other two modes of 10 and 20 revolutions. Consequently, it is concluded that utilizing Ag-HEG nanofluids in conjunction with the four-blade turbulator featuring 30 revolutions is the optimal choice for improving heat transfer in conical spiral tube heat exchangers while maintaining an acceptable level of pressure drop. This combination outperforms traditional fluids and turbulators.