External Wall Heat Research Articles

Numerical analysis of flame instabilities in narrow channels: Laminar premixed methane/air combustion

Premixed flames propagating within small channels show complex combustion phenomena that differ from flame propagation at conventional scales. Available experimental and numerical studies have documented stationary, non-stationary, or asymmetric modes that depend on properties of the incoming reactant flow as well as channel geometry and wall temperatures. This work seeks to illuminate mechanisms leading to symmetry breaking and limit cycle behavior that are fundamental to these combustion modes. Specifically, four cases of lean premixed methane/air combustion—two equivalence ratios (0.53 and 0.7) and two channel widths (2 mm and 5 mm)—are investigated in a 2D configuration with constant channel length and bulk inlet velocity, where numerical simulations are performed using detailed chemistry. External wall heating is simulated by imposing a linear temperature gradient as a boundary condition on both walls. In the 2 mm channel, both equivalence ratios produce flames that stabilize with symmetric flame fronts after propagating upstream. In the 5 mm channel, flame fronts start symmetrically, although symmetry is broken almost immediately after ignition. Further, 5 mm channels produce non-stationary combustion modes with dramatically different limit cycles: in the leaner case ( φ = 0.53), the asymmetric flame front flops periodically, whereas in the richer case ( φ = 0.7), flames with repetitive extinctions and ignitions (FREI) are observed. To further understand the flame dynamics, reaction fronts and flame fronts are captured and differentiated. Results show that the loss of flame front symmetry originates in a region close to the flame cusp, where flow and chemical characteristics exhibit large gradients and curvatures. Limit cycle behavior is illuminated by investigating flame edges that are formed along the wall, and accompany local or global ignition and extinction processes. In the flopping mode ( φ = 0.53), local ignition and extinction in regions adjacent to the wall result in oblique fronts that advance and recede along the wall and redirect the flow ahead of the flame. In the FREI mode, asymmetric flames propagate much farther upstream, where they experience global extinction due to heat losses, and re-ignite far downstream with opposite flame front orientation. In both cases, an interaction of flow and chemical effects drives the asymmetric limit cycles. The lack of instabilities and asymmetries for the 2mm cases is attributed to insufficient wall separation, which is of the same order of magnitude as the flame thickness.

Properties of Bricks and Masonry of Historical Buildings as a Background for Safe Renovation Measures

This article is the first approach of research aimed to recognize historical building materials and their properties, analyzing safe and effective renovation measures to decrease energy consumption for heating of buildings built before 1940. Brick samples taken from historical buildings were tested to assess their compressive strength, soluble salt content and water absorption. In addition the external wall heat transfer coefficient was determined and the moisture level in masonry was set in two different climatic conditions.

Study on Heat Insulation Performance of External wall of Low Temperature Grain Storage Granary

Based on a low temperature grain storage granary in Beijing zone, an unsteady two-dimensional model of heat transfer on external wall is set up in the study. The heat-transfer model is simplified reasonably. According as the heat-conduction differential equation, initial and boundary conditions, the temperature fields of external walls in south with different kinds of heat insulation are simulated and calculated with CFD software. The numerical results are compared and analyzed. The results show that the attenuating role of external wall with interior heat insulation to the outdoor air temperature fluctuation is smaller than which with exterior heat insulation and that the heat storage capacity of external wall to prevent of the temperature fluctuation is weaker, too. At last, it is pointed that external wall heat insulation offers a guarantee to achieve low temperature grain storage and energy-saving and the heat insulation performance of external wall with exterior heat insulation layer is better than which of interior heat insulation layer and during the energy-saving design of the granary external wall, the technology of exterior heat insulation should be used as far as possible.

Study on two-phase flow instabilities in internally-ribbed tubes by using frequency domain method

A new mathematical model was proposed in this paper based on the frequency domain theory to investigate two-phase flow instabilities in internally-ribbed tubes. Two significant improvements were made to conventional models, one of which was that the heat storage of the tube wall metal, the internal wall heat flux and the external wall heat flux were considered as dynamic parameters, and another one was that a new feedback relationship was established. The new model was verified by comparing the prediction results to both the corresponding experimental data in literature and the results obtained with SIMULINK. Then the two-phase flow instabilities in internally-ribbed tubes were studied systematically. It was shown that the stability of two-phase flows increased with pressure, mass flux or inlet resistance coefficient. It was also found that there existed a worst point of the stability (WPS) of the two-phase flows when the inlet fluid subcooling increased. Two-phase flows in the 6-head ribs internally-ribbed tube (ϕ38.1 × 7.5 mm) was more stable than that in the 4-head ribs internally-ribbed tube (ϕ28 × 5.41 mm). The present model and results could provide guidance for the design and operation of boilers.

Study on Effect of the External Wall and Windows Heat Load Ratio to the Total Heat Load on the Indoor Heat Comfortability

To study the relation between the heat load ratio of the external wall and window to the total heat load and the indoor heat comfortability index PMV, putting up the numerical calculation to a radiator heating room. It gains the value of the PMV when the heat load ratio of the external wall and window to the total heat load is separately 0.2 and 0.8, 0.3 and 0.7, 0.4 and 0.6, 0.5 and 0.5, 0.6 and 0.4. And it draws the curve about the PMV and the different heat load ratio of the external wall and window to the total heat load. The curve shows that when the heat load ratio of the external wall and window to the total heat load is 0.4 and 0.6, the indoor heat comfortability index PMV is maximal.

Analysis of Heat Transfer by Thermal Bridge of Corner Wall in a New Glass Regenerated Pumice External Wall System

This paper was based on heat insulating members made by a new type of waste glass regenerative pumice, with the characteristics of light weight, permeable water retention, ventilation, heat insulation, fire retardant, etc. In the external wall heat insulating system of building envelops, the thermal bridge of external wall corner was simulated with ANSYS. The temperature field and flux field distribution will be shown respectively of non-insulation, self-insulation, internal insulation and external insulation. Through comparing the heat preservation of different insulation forms, the external insulation was better than internal insulation and self-insulation. Besides, considering the imperfection of calculating the wall average heat transfer coefficient in current energy saving standards, a simplified method was presented to calculate the linear heat transmission coefficient with ANSYS. The two coefficients were compared to serve the latest research.

Vortex structure-based analysis of laminar flow behaviour and thermal characteristics in curved ducts

This paper presents a numerical investigation for examining the secondary vortex motion and associated heat transfer process in fluid flow through curved passages. Overcoming current modelling limitations, the study formulates an improved simulation model based on 3-dimensional vortex structures for describing secondary flow and its thermal characteristics. For developing laminar fluid flow through curved rectangular ducts, the analysis performs a detailed parametric study involving the contours of helicity and outer duct wall pressure gradient for a range of flow rates, duct aspect ratios, duct flow curvatures and external wall heat fluxes. The flow conditions leading to hydrodynamic instability and Dean vortex generation in curved passages are carefully analysed, identifying the flow and geometrical parametric influences. Active interaction of buoyancy force on fluid motion arising from wall heating is considered where aspects of boundary layer separation is used in recognising thermal enhancement due to secondary flow. The simulated results are validated with the available experimental data. Surpassing precision and reliability of current practise, the study develops two numerical approaches offering improved capabilities for capturing the onset of hydrodynamic instability and resulting Dean vortices. These techniques are compared and assessed for their merits of application in determining the onset of secondary flow hydrodynamic instability with and without external wall heating.

Experimental investigations of the quenching phenomena for hemispherical downward facing convex surfaces with narrow gaps

To investigate a gap cooling mechanism between a corium and a reactor pressure vessel, quenching phenomena of hemispherical downward facing surfaces with narrow gaps with respect to wall heating conditions have been investigated experimentally. The experiments were performed with a hemispherical gap thickness of 1 and 2 mm, and the wall heating conditions, that is, with and without an external wall heating. That is, the internal vessel (or both internal and external vessels) was heated up to 450 °C, and distilled water was supplied to the hemispherical gap region, and then the temporal histories of the internal and the external vessels were measured by the thermocouples embedded in the internal and the external vessels. From the interpretations of the wall temperature and the heat flux history, it was found that the quenching process was changed by the wall heating conditions. If the external wall was not heated, then the internal vessel wall was stably quenched from the bottom to the top region because the coolant water was supplied to the gap region through the cold external wall. However, if both the internal wall and the external wall were heated, then the quenching phenomena were more complicated and slower as the gap thickness decreased, because the coolant supply to the gap was restricted.

Entropy generation and optimal analysis for laminar forced convection in curved rectangular ducts: A numerical study

The present paper analyzes entropy generation induced by forced convection in a curved rectangular duct with external heating by numerical methods. The problem is assumed as steady, three-dimensional and laminar. The local entropy generation distributions as well as the overall entropy generation in the whole flow fields, including the entrance region and fully developed region, are analyzed. The effects of three important factors, including Dean number, external wall heat flux and cross-sectional aspect ratio, on entropy generated from frictional irreversibility and heat transfer irreversibility are investigated separately in detail. The results show that the major source of entropy generation in the flow fields with larger Dean number and smaller wall heat flux comes from frictional irreversibility; whereas for the flow fields with smaller Dean number and larger wall heat flux the entropy generation is dominated by heat transfer irreversibility. The competition between the entropy generation from the irreversibility caused by fluid friction and heat transfer is complicatedly related with the three factors, making the relationship between the resultant entropy generation and the three factors non-monotonous. Based on the minimal entropy generation principal, the optimal condition can induce the minimal entropy generation in the flow fields. Through the optimal analysis, the optimal aspect ratio is found to be dependent on heat flux and Dean number. For each case with specific aspect ratio and wall heat flux, there exists an optimal Dean number, and the optimal Dean number is found to increase with wall heat flux. The detailed optimal analysis is provided in the present paper, which is worthwhile for heat exchanger design from the second law of thermodynamics since the thermal system could have the least irreversibility and best exergy utilization if the optimal Dean number and aspect ratio can be selected according to the practical design conditions.

Numerical prediction of secondary flow and convective heat transfer in externally heated curved rectangular ducts

A numerical simulation is presented to describe the secondary flow characteristics in the flow through curved rectangular ducts that are heated on the outer (concave) wall. The buoyancy body forces due to external heating are incorporated in the formulation. The governing equations of momentum, mass continuity and energy of the flow are non-dimensionalised and transformed into vorticity-streamfunction. The equations are discretised using a finite volume method and the dynamical variables are defined on a staggered grid. The resulting penta-diagonal system of linear algebraic equations is solved using the Strongly-Implicit-Procedure. Numerical computations are performed for the flow through rectangular ducts of aspect ratios 1 to 8 and for the Dean number lying in the range 20 to 500. The external wall heat fluxes of 0 to 200 W·m −2 are considered. The computation accurately captures the influence of external heating on the secondary vortex motion and, predicts the occurrence of Dean vortices in non-isothermal flow through the curved ducts. With wall heating, the flow pattern rapidly deviate from the symmetrical vortex motion observed under isothermal conditions. The formation of Dean vortices still takes place in the flow although it is postponed to higher flow rates where the flow symmetry is gradually re-established. The number of Dean vortices formed in the flow is strongly influenced by the aspect ratio of ducts. Convective heat transfer is significantly enhanced by the secondary flow particularly when the Dean vortices emerge at the outer wall. The predicted characteristics of secondary flow and heat transfer conform to and agree well with the available experimental data.

Designing Solar Hot Water Systems for Scaling Environments

Component failures and system performance degradation in SHW systems due to scaling are common in areas with hard water. It appears that many valve and pump failures on the potable water side are related to scaling, and any scale build-up on heat transfer surfaces will result in performance degradation. Different designs are compared in regard to their susceptibility to problematic scaling. Indirect systems utilizing external and tank wall heat exchangers are compared in regard to the rate of scaling and the consequences of scaling on system performance. The tank wall heat exchanger appears preferable over a doubly pumped external heat exchanger, both in terms of system reliability and resistance to performance degradation.

Analysis of unsteady forced convection in turbulent duct flow

The results of a theoretical analysis and experimental study of unsteady forced convection with turbulent flow in the thermal entrance region of a circular duct, subjected to a periodically varying inlet temperature, are presented. The related unsteady energy equation for the thermal entrance region is analytically solved under a general boundary condition of the fifth-kind, accounting for the effects of both external convection and wall heat capacitance. Together with empirical models for expressing both eddy viscosity and turbulent velocity profiles, the generalized integral transform technique is used to provide hybrid analytical-numerical solutions for the thermal response of the fluid with a prescribed accuracy. An experimental setup was built and used to validate the mathematical modeling wherein the thermal response of the fluid is obtained in terms of amplitudes and decay indices with respect to the inlet condition for fully developed turbulent flow, due to a sinusoidal variation of the inlet temperature. Satisfactory agreement prevailed between the theoretically and experimentally determined heat transfer characteristics for a succession of axial positions, thereby establishing the theoretical model and numerical technique. To further enhance the model's practical applicability to engineering problems, the analytical technique is extended in obtaining results on the effects of the modified Biot number, fluid-towall thermal capacitance ratio, and the Reynolds number, on the thermal response of the fluid within the temperature field, which are presented in tabular forms.

Computer-aided analysis of fluidized bed to external wall local heat transfer

Fluidized bed to external wall heat transfer phenomena play an important role in the design and rating of a fluidized bed. In fluidized beds where the heat exchanging surfaces are located along the external walls, maximum heat transfer rates are desirable with a rather uniform distribution over the wall height. Although available models rely upon local hydrodynamic information such as particle velocity and trajectory, gas velocity and void fraction, they cannot provide specific information about these parameters. Instead, the use of global information, derived through certain assumptions or limited analytical expressions, makes the predictions of these models far from realistic. The many empirical equations can provide only average heat transfer rates along the heat exchanging surface and usually yield widely different results. In this study, a computer program was developed to calculate the local bed-to-wall heat transfer, when coupled to the CHEMFLUB computer program which predicts the local hydrodynamics of a fluidized bed [1]. The new program enables the user to adjust various design parameters until a more satisfactory design is reached or to evaluate the heat transfer in the existing beds. This paper is a brief summary of the various heat transfer models and gives sample results for two design cases: a shallow and a narrow bed. Comparisons with available experimental data and correlations are also provided to evaluate the proposed computer model.