Mass flow maldistribution among parallel tubes of molten-salt solar tower receivers can modify heat transfer, tube temperature and stress, affecting receiver durability. Although previous studies have analysed receiver lifetime and, separately, thermo-hydraulic effects of mass flow maldistribution, its impact on creep-fatigue lifetime has not yet been quantified. This work presents the first tube-resolved lifetime assessment of a molten-salt external receiver accounting for maldistribution arising from the hydraulic interaction between inlet and outlet headers and parallel tubes. A hydraulic network model is coupled with a fine-grid thermal model resolving every receiver tube, and a thermo-mechanical model including stress relaxation and creep-fatigue damage. Results are compared with the uniform-flow assumption. The implications for a representative-tube model and minimum-flow assumption are also discussed.The predicted mass flow rate distribution remains nearly unchanged throughout the day for a given panel, with variations below 0.5% in the normalised profile of the inlet panel, whereas differences around 0.87% arise along the receiver flow path owing to the salt thermophysical properties evolution. The redistribution enhances the internal heat transfer coefficient around the central tubes, partially compensating for the non-uniform absorbed solar flux and reducing tube wall temperatures by up to approximately 0.6%. However, these temperature reductions do not necessarily reduce creep damage because of the coupled effects of stress relaxation and elastoplastic redistribution. Accounting for mass flow maldistribution modifies the predicted panel lifetime and the location of the most critical tube. For the first four panels, the detailed distribution model predicts lifetimes 0.967–0.997 times those obtained under the uniform-flow assumption, with the largest reduction occurring in Panel 1, which remains far from limiting. Moreover, the minimum-flow representative-tube case analysed is not systematically conservative compared to tube-resolved models. These results demonstrate that mass flow rate maldistribution should be considered in detailed lifetime assessments of molten-salt solar receivers.
Flexible operation subjects coal-fired utility boilers to distinct but interacting combustion, emissions and equipment-safety constraints at low load and during rapid load changes. This review synthesises 203 publications on sensing, prediction, dynamic modelling, optimisation and system integration, linking operating problems to measurements, model resolution and validation requirements. Low-load operating points are better documented than plant responses during ramps. Low-load assessment centres on ignition, burnout and heating-surface conditions; ramping also requires attention to fuel delivery, steam/water dynamics and selective catalytic reduction. Low-order dynamic models support load-tracking analysis, whereas spatially resolved or coupled models address local heat transfer and tube-wall constraints. Validation must distinguish steady regression, transient forecasting and control, and test transfer across relevant fuels and operating conditions. Predicted optimisation gains require independent evaluation before they support claims of plant benefit. For plant application, selected settings must be linked to operating constraints and measured responses, with benefit attribution accounting for fuel and load differences and uncertainty. System integration, physical implementation and repeated automatic feedback remain distinct. Future work should combine complementary measurements with task-appropriate models and independently evaluate the resulting operating actions.
The drying section of the paper industry discharges large volumes of high-temperature and humid air containing substantial sensible and latent heat, while conventional fin-and-tube heat exchangers are susceptible to fouling and corrosion under these conditions. Existing air-side Nusselt number correlations for pillow-plate heat exchangers (PPHE) have primarily been developed for single-phase, non-condensing flow, leaving their heat transfer characteristics under condensing paper-exhaust conditions insufficiently characterized. To address this gap, this study investigates condensing PPHE performance in the low-Reynolds-number and transitional regimes and develops a geometric correction approach to account for changes in flow cross-sectional area. A PPHE was experimentally evaluated under simulated paper-exhaust conditions at inlet temperatures of 70–90 °C, relative humidity of 25–45%, and air flow rates of 200–600 m3/h. Two test sections with different plate numbers (7 and 3 plates) were used to cover Reynolds numbers of approximately 700–3600. The high-velocity configuration exhibited an overall heat transfer coefficient up to 2.3 times higher, whereas the low-velocity configuration achieved up to 1.8 times greater total heat recovery owing to its larger condensing area, with latent heat accounting for up to 77% of the recovered heat. A Nusselt number correlation with a Reynolds number exponent of 0.48 was developed and extended to both geometries using an empirical flow-area correction factor, reducing the mean absolute percentage error from approximately 16% to 3.4%. These results demonstrate that the overall heat transfer coefficient and total heat recovery should be evaluated independently under condensing conditions and provide a practical correlation for designing PPHE for humid-exhaust heat recovery.
This study compares the apparent gasification reactivity and conversion-dependent microstructural evolution of petroleum coke (PC) under CO2 and H2O-containing atmospheres at 1000 °C using a micro fluidized-bed reactor with online particle sampling system. Under the investigated conditions, the H2O-containing case reached complete conversion within approximately 80 min, whereas the CO2 case reached about 40% over the same period; this rate difference is condition-dependent because reactant partial pressures were not matched. N2 adsorption showed rapid early development of accessible porosity followed by surface-area loss at higher conversion, with larger BET areas under H2O at equivalent conversion. FTIR indicated progressive relative changes in oxygen- and hydrogen-containing functionalities. XRD and Raman showed increasing carbon ordering in both atmospheres, but the increase was more pronounced under CO2, while H2O-gasified residues retained a larger contribution of disordered carbon. These results establish atmosphere-dependent links among apparent reactivity, pore evolution, surface functionalities, and carbon ordering in PC.
Earth-Air Heat Exchangers (EAHEs) are low-energy thermal management technologies with simple design and low operating and maintenance requirements. However, commercial-scale experimental research on their performance and soil thermal response remains limited, particularly for horizontal parallel designs in cold climates, where existing assessments are predominantly simulation-based. This study investigates a horizontal parallel EAHE in a commercial solar greenhouse in Ontario, Canada, evaluating an as-built multi-pipe array at the inlet/outlet manifolds as a complete system, measuring air and soil sides simultaneously, and converting depth-resolved soil temperatures into layer-wise stored energy.During an 11-day July experiment, greenhouse conditions, EAHE inlet and outlet temperatures, control signals, and soil temperatures at multiple depths and positions near a buried pipe were recorded every 6 min while the EAHE ran on a fixed twice-daily schedule. Soil temperature profiles were reconstructed by least-squares quadratic fitting to quantify heat storage, and performance assessed using inlet-outlet temperature differences, coefficient of performance (COP), and matched 4.5-day periods with and without EAHE operation.Afternoon cooling produced inlet-outlet differences near 7 °C and a maximum COP of 9.73, averaging 7.11, consistent with other measured greenhouse installations. Early-morning heating reached a maximum COP of only 1.25 because of small air-soil temperature differences. Soil adjacent to the pipes charged during afternoon operation and retained heat between cycles, indicating short-term load shifting, while near-surface soil followed greenhouse air temperature and solar gains. Fixed-schedule operation reduced ventilation heat loss by only 5.7% while increasing total electrical use; real-time temperature-based control is therefore recommended. The dataset is published openly for model validation.
