Browsing by Author "Hammond, G. P."

Now showing 1 - 5 of 5
  • Thumbnail Image
    ItemOpen Access
    Convective heat transfer within mechanically-ventilated building spaces
    (Cranfield University, 1984-04) Alamdari, F.; Hammond, G. P.
    A hierarchy of interacting and interdependent approaches have been developed for calculating internal surface convective heat transfer coefficients within mechanically-ventilated rooms. A 'high-level' computer code is developed for non-bucyant and buoyant flow based on the "elliptic' code of Pun and Spalding (1977), in which 'upwind' finite-difference approximations to the governing partial-differential equations for continuity, momentum and thermal energy are formulated in terms of 'primitive' pressure-velocity variables. Closure of these time-averaged, elliptic equations is obtained via transport equations for both the turbulence kinetic energy and its dissipation rate. The high-level code solves the difference equations for a predetermined size, staggered grid in an iterative 'line-by-line' manner using a guess-and-correct procedure. An 'intermediate-level' computer code (the ROOM-CHT program) has also been developed for the above purpose, which employs 'informed" estimates of the flow and thermal field based on the known mean flow properties of wall-jets. The corresponding heat transfer distribution across the room surface is calculated using wall-jet profile analysis or improved data correlations for bucyancy-driven convection as appropriate. Caqputations are presented for a room into which air is injected through a low or high side wall register. The supply of air governed by both cyclic and modulating control was examined. The intermediate-level code is advocated as being the most appropriate for meeting the requirements of dynamic building thermal models. This code was verified by comparison with the high-level code and with experimental measurements. The oomputed heat transfer coefficients from the intermediate-level code were found to be in good agreement with that of the high-level code. Both indicate significantly higher values than those which would be obtained from established design guides. These high values suggest errors in building thermal models based on guide data, including substantial under-estimation of preheat times.
  • Thumbnail Image
    ItemOpen Access
    The effect of nozzle inclination on heat transfer in jet impingement systems
    (Cranfield University, 1981-09) Oladiran, M. T.; Ward, J.; Hammond, G. P.
    Jet impingement heating and cooling techniques are used extensively in industrial applications. in some of these installations, the axis of the jet can be inclined relative to the impingement surface. The impingement flow is then unsynLmetrical so that the heat transfer rates are modified. At present, there is a lack of information concerning the effect of inclination on jet impingement heat transfer. Thus, the experimental study reported in this thesis is primarily concerned with the measurement of local and average heat transfer coefficients associated with the impingement of inclined turbulent circular jets onto flat plates. A single free jet exiting into initially stagnant surroundings was considered and the nozzle inclination was varied from 300 to 900 to the surface. The tests covered the range: Z/d (nozzle-target separation) of 6 to 16 and Re (jet Reynolds number based on exit conditions) of 32500 to 65000. The effect of the exit nozzle shape was also determined. In multiple jet systems, the flow from the upstream jets can significantly affect those situated in the downstream section. Thus, the effect of nozzle inclination on the performance of an impinging jet exiting into a cross flow was also investigated. Thus, as well as the angle of inclination (a), the magnitude of the cross flow (Uc) and the width of the duct (H/d) were also altered in this confined situation. The ranges of these variables were 300
  • Thumbnail Image
    ItemOpen Access
    Experimental heat/mass transfer studies of turbulent wall-bounded jets associated with mechanicallly ventilated enclosures
    (Cranfield University, 1986-03) Montazerin, N.; Hammond, G. P.
    The development and use of a'n experimental test rig is reported which is capable of modelling two- and three-dimensional wall-bounded air jets. This test rig was primarily produced in order to facilitate the experimental verification of computer codes for calculating convective heat transfer within mechanically- ventilated enclosures. Special attention was therefore given in the design of the rig to heat transfer measurements within such enclosures. The analogy between heat and mass transfer and the application of the naphthalene sublimation technique are explained. Also use of phase change paints in heat transfer measurements in general, and wall-jets in particular, is discussed and experimentally demonstrated. The boundary conditions for the application of each of the above two methods are then specified. The mass transfer method may be used for the case of a heated plate and a jet at ambient temperature while the phase change paint method is applicable to a heated jet. Heat/mass transfer studies are carried out for two different geometries. First beneath a plane wall-jet- obstructed by a normal flat-plate (Alamdari, Hammond and Montazerin (1986 bound paper)), where the data are compared with the computations of the 'intermediate-level' convection model of Alamdari and Hammond (1982) and the high-level 'elliptic' finite domain flow model of Pun and Spalding (1977). The comparison has been a clear demonstration of the capabilities of the computer codes and has shown that although their results over flat surfaces are in good agreement with the test data, their predictions for jets flowing round corners need further research. Secondly the flow and heat transfer characteristics of a three-dimensional jet parallel to a flat plate has been studied. In this case the flow field and mass transfer are modelled and an equation is finally derived which estimates the average heat transfer over a plate parallel to a bluff-jet for a variety of off-set heights, Reynolds numbers and nozzle aspect ratios and can readily be used by design engineers handling such flows.
  • Thumbnail Image
    ItemOpen Access
    Improved convective heat transfer and air infiltration models for building thermal simulation
    (Cranfield University, 1985-06) Melo, C.; Hammond, G. P.
    10 Intermediate-level'o computer codes are advocated as being the most appropriate for meeting the requirements of dynamic building thermal models. Such codes may be developed via the .4 computer-generalizationA Of analytical solutions and data correlations, which are then verified using higher-level ccoputational procedures and/or experimental measurements. Two intermediate-level ccniputer codes are described: one to model the convective heat exchange at the external facades of a building (WIND-CHT program), and the other to calculate the hourly mean rates of air infiltration into buildings (FLOW program). These codes take into account most of the key parameters such as wind speed and direction, the change in shape and height of the atmospheric boundary-layer over different terrains, the relative dimensions of the building,, the indoor-outdoor temperature difference and the leakage characteristics of the building. Both the WIND-CHT and FLOW programs are carpared with field experimental data, and good agreement is shown. The sensitivity of two dynamic building thermal models to the external convection and air infiltration input data are then assessed. The NBSLD (National Bureau of Standards Load Determination) 'response factor' program (1981) and the BM (British Research Establishment) 'admittance procedure' program (1984) were chosen for this purpose. The sensitivity of these models to the internal convection input data was also assessed. In this case the ROOM-CHT program, developed by Alamdari and Hammond (1982) was employed. Both models displayed a considerable variation in their results when the 'traditional' input data were replaced by the 'improved" values, although the extend of the impact of the convection and infiltration models is likely to depend on the conditions prevailing in and around the particular building being simulated.
  • Thumbnail Image
    ItemOpen Access
    Space air-conditioning of mechanically-ventilated rooms : computation of flow and heat transfer
    (Cranfield University, 1986-06) Mohammad, W. S.; Hammond, G. P.; Alamdari, Fariba
    Computational studies of two- and three-dimensional, turbulent recirculating flows within mechanically-ventilated enclosures are reported. Two principal cases are examined: (i) two-dimensional offset jets: and (ii) three-dimensional flow induced in rooms by supply jets emanating from low or high side-wall registers. The calculations were undertaken using iterative finite-domain proceedures which solve the conservation equations for mass, momentum and enthalpy, together with additional transport equations for the turbulent kinetic energy and its dissipation rate . The effect of buoyancy waS. explicitly accounted for when modelling these equations, in order that they could be employed to simulate buoyant flow in ventilated rooms. Computations of the mean velocity, temperature and convective heat transfer distribution are reported, and compared with experimental data where available. A modified version of the two-dimensional elliptic code of Pun and Spalding (1977) was employed to simulate the offset jet case. These involve the discharge of a turbulent jet parallel to a flat surface and eventually attaching to it. The investigations covered a wide range of offset ratio (3.5-32.4). and the computed flow properties are compared with measurements from several sources. These comparisons show good agreement downstream of the reattachment point, while some discrepancies are evident upstream from this location. The differences therefore occur mainly in the recirculating flow region, and are believed to arise from shortcoming in the starting profiles, the turbulance model and the treatment of the near-wall flow. A three-dimensional elliptic finite-domain code was developed to simulate the complex, jet-induced flow within rectangular enclosures. The code was verified using both laminar and turbulent flow test cases on simpler geometries. Comparisons with the measurements and predictions reported by previous researchers were employed for this purpose. Subsequentlyr the ventilated room simulations were undertaken using three different ventilation arrangements with thermal conditions corresponding to isothermall non-buoyant (constant property) and buoyancy"affected flows. The computations were again compared with experimental and numerical predictions of previous researchers. This comparison displayed generally good agreement with these sources. A study of the flow and convective heat exchange within a warm-air heated rom, for which buoyancy effects are significant# is also reported in a bound paper (Alamdari, Hammonda nd Mohammad, 1986) for three different heat loads. Its aim to assess the balance between accuracy and economy provided by the present higher-level method compared with the intermediate-level convection model of Alamdari and Hammond (1982) when used to supply building thermal simulation programs with accurate convection heat transfer data. The computed results of both models were compared, and indicate that the intermediate-level is a valuable alternative source that can satisfy the needs of building thermal modellers. It provides resonable accuracy at a very modest cost in computing terms.